This looks like fun

Okay, let's go through my currently-in-play 3.5 characters. I suspect that at least one of these is trivial to convert, and at least two are very difficult, but we'll see what others think.
1. A battle-priestess of the goddess of fire, wrath, arguments and war. She honors her deity by devoting her life utterly to her chosen crusade - scouring the world clean of the fiends and undead who plague it. She is knowledgeable about such foul creatures, and trained in special techniques to battle them more effectively (in 3.5, she is a sacred exorcist with chosen foe evil outsiders, and high ranks in Knowledge (Planes) and (Religion)). She is a skilled warrior who fights with a simple quarterstaff (her deity's favored weapon), imbued with divine power by her goddess' holy fury - her fighting style relies on speed and many quick jabs rather than a few overwhelming blows (she uses her staff as a double weapon). She can reduce undead to dust by channelling sun-fire and command creatures of fire to do her bidding (her temple is guarded by commanded pyrohydras). Fire does not burn her, and when she wishes to travel quickly, she can transform herself into dark ashy smoke and let the wind carry her. As a servant of the Blazing Wrath, she can unleash powerful spells drawing on elemental fire/light and the divine Word (since her deity is patron of both fire and arguments) - in 3.5 she often used fire/light spells like Fire Seeds and Bolt of Glory, and sonic spells like the Power Words, Holy Word and Lion's Roar. She can infuse her allies with holy rage, while also using her divine powers to heal wounds and keep her allies fighting at peak efficiency (her goddess delights in battle and favors the brave). Out-of-combat, she often calls on her goddess' blessings to heal the sick and wounded and provide food to the starving, as it is not only adventurers who fight the war against the darkness. She is currently L15 (so it's okay if you need to use a L30 4e character to emulate her). Her weaknesses are a lack of powerful mobility + battlefield control magic (although she fills the 4e "controller" role decently, just because she has some huge AoE spells). In terms of 4e roles, she really has elements of all four, but I guess she's more a leader than anything else - she has excellent party buffs and healing, powerful AoE spells (both damage-dealing and inflicting status effects, just not much battlefield control), powerful single-target damage spells (but not the mobility strikers are meant to have), and usually has the highest AC in the party by a fair margin (but isn't very "sticky" in melee, as 4e controllers are meant to be).
2. A young sorcerer who is so strongly attuned to magic that her conscious and unconscious wishes become reality. She dreamed of flying, and so she can fly (and so can her friends, because she's a generous person and likes to see her friends happy); when she wanted to visit her friend in another country, the magic took her there. When her pet dog died, her loneliness formed horses out of the stuff of magic itself. When she is afraid, freezing mists, blinding dust clouds and the spirits of the dead coalesce to protect her, and the dark shadows of her nightmares reach up from the earth to clutch at her enemies. Doors open for her, lights appear when she wants them, and she and the area around her are always immaculately clean. She can sense magic all around her, and if she does not wish another's spell to complete, it usually doesn't. She can play with raw elemental magic (cold, fire, electricity, shadow, force), shaping it as she wishes. People, creatures and magic items are all friendly to her as a default (she wants to be liked, and the magic makes it so). The magic protects her - if she falls the winds will always catch her, and chance tends to skew in her favor. Arrows and spells fired at her tend to randomly strike other people instead. If she's willing to sacrifice a bit of her life-force to let her talk directly to the magic, she can do nearly anything (Limited Wish). She's a L15 sorcerer / wild mage / fatespinner with a focus on mobility magic, battlefield control and (mostly single-target) debuffing. She likes seeing her friends get to show off (it makes them happy), so she often spends her swift actions in combat (sometimes even standard actions) 'porting her allies into full attack range or casting buffs/debuffs to help her friends' spells work. She's also pretty good at single-target elemental direct damage (fire/cold/electricity/force). She has a LOT of metamagic and being able to reshape and alter her spells is important to her. Skill-wise she has excellent Concentration, UMD and Spellcraft, pretty good Knowledge (Arcana) and (The Planes), and decent Charisma-based skills across the board despite being untrained in them (most are +13 or +15 untrained, Diplomacy and Gather Info are +18 untrained). She's quick and somewhat hyperactive, bright but not brilliant, without much physical strength or stamina and no common sense whatsoever - but her charisma and leadership ability are such that there's a tribe that worships her as a goddess. In terms of 4e roles, she seems to be mostly controller/striker with a dash of leader - she's definitely not a defender.
2a. The previous character's cohort - a sage and skald from the frozen north, who has spent his life studying different perspectives on magic and fitting them into a seamless whole. Some say that magic is like a song; some say it is like a tapestry; some say it is drawn from blood, or the stars... he has studied all these, and incorporated them into his own unique style of magic. A lyrist of extraordinary talent, his wintry music-magic hums through the strummed threads of the Weave, granting extra power to his allies' spells even as it helps warriors strike true, imbues their weapons with magical frost, helps them dodge their enemies' blows, grants them healing, cloaks them in illusion rendering them invisible, makes them immune to most impediments, and moves them through the Weave to the most advantageous position. The character is a L13 bard/wizard/warweaver/sublime chord (yes, it's complicated, requiring material from about five books, and there is no WAY you can pull this off with 3.5 Core - pure bard comes closest). His first round of combat usually involves giving the entire party +5 to initiative when we roll init, then on his turn giving everyone +4 Charisma (morale), +4 to hit (morale), +1d6 cold damage on melee attacks, fast healing 1, +4 AC (insight), +2 CL, +2 save DCs, Greater Invisibility, Freedom of Movement, and Greater Dimension Door. He's most definitely a leader, although he has some striker-controller abilities just for emergencies (Telekinesis, and he can borrow the sorcerer's runestaff containing Otto's Irresistible Dance). He's smart and very charismatic, but all his other stats are mediocre (he had to use elite array, so yeah). He's not actually much of a diplomat, having spent most of his life studying magic - he's quite knowledgeable, and has picked up all sorts of odd crafts along the way (he's a competent astrologer and weaver as well as a very skilled lyrist).
3. An elven swordsman, trained as a ranger in a mixed wild elf / wood elf tribe, but long absent from the woods, who has dedicated himself to mastering the sword. He is keen of eye and ear and a skilled tracker (high Spot/Listen/Survival, and the Track feat). His chosen sword style emphasises a few powerful, elegant cuts rather than a flurry of weaker strikes, and using speed and footwork to dart in, eviscerate your foes and then retreat to a better position. This is not cowardice - fighting on your enemies' terms is foolish. He especially abhors the undead, and worships Pelor the Sunfather (this is mostly fluff, although he does have Favored Enemy Undead from his ranger level). He is a L15 wood elf fighter/ranger/barbarian/dervish, and his combat style is straightforward - he clicks on his boots of speed (giving him 70ft movement), activates his Dervish Dance, and full-speed-tumbles in, full-attacks the bad guy and full-speed-tumbles back out (rinse and repeat on later rounds). He uses a falchion with Power Attack and Improved Critical - he is NOT a two-weapon-fighter. He has superb jump/tumble skills that generally allow him to go anywhere on the battlefield unhindered (he has a ring of freedom of movement for those pesky battlefield control spells), and often permit him to take advantage of openings in his foes' defences (he has the Acrobatic Strike feat). 4e role-wise, he seems to be pretty much the archetypal striker.
4. A gnome shadow-shaper whose power comes from draconic heritage, and who is becoming more and more dragon-like (and also tainted with shadow) as she explores her inborn powers. She can infuse her allies (and herself) with draconic power, granting them flight, speed, extra attacks and the ability to see perfectly in darkness (her understanding of shadow-magic is such that they can even see through magical darkness); if her allies do not need to fly, she can become airborne in an instant rather than spending time drawing forth the dragon-power, but such flight is draining to maintain for more than a few seconds. She can draw on her own draconic ancestry to make herself scaly, tougher, and resistant to energy for long periods of time. Like her dragon ancestors, she has power over the earth - she can create mud slicks and clouds of blinding dust to impede her foes. Her experiments with the magic of shadow and darkness have leached all the color from her, but granted her the power to see far better than normal in shadowy illumination (she has Superior Low-Light Vision), to cloak herself in shadow, to travel swiftly by jumping through shadow, to conjure up life-sucking darkness that drains the strength and will from foes, to banish her enemies to a lightless pocket plane, and to shape realistic illusions from light and shadow. She is keen of eye and silent of tread, and almost impossible to see in the shadows. (She has Hide +27, Move Silently +20, and Spot +18.) She is currently a L10 sorcerer / ruathar / shadowcraft mage. She's smart, acerbic, tough as nails, charismatic in the spooky scary sense, and can barely lift her Heward's Handy Haversack.
5. A storm-worshipping, bat-riding warrior-druid who fights in defence of the natural world. Raised in the jungles of the far south, she was a member of a shamanic cult that preserved the lore of the ancient bat-people who once dwelt in that land. The shamans rode sacred dire bats (considered to be messengers of the gods) and served as intermediaries between the tribes and the spirit world. The PC was taken from her tribe by slavers, and on her escape found herself in a strange new northern land, but she still strives to uphold her beliefs, and her faith was rewarded when one of the sacred bats appeared in response to her summoning ritual. It was a storm that freed her from the slavers' ship, so she worships the gods and spirits of storm and sky, and she strikes like a lightning-bolt from the heavens against those who would defile nature. She prefers to fly, mounted on her bat companion, whenever possible - she can make devastating charges from above, using her lance (with is engraved with a lightning bolt and the runes for lightning), and in time she will be able to invoke the powers of the storm to strike multiple times on a single charge, like a forked lightning bolt. Her spells are mostly self-buffs, with some healing - her service to nature is rewarded with superhuman vision and hearing, regenerative abilities (shared by her bat), speed and endurance for her bat, and sometimes short bursts of enhanced strength. In some ways the best way to think of her is as a True Neutral Paladin of Nature - mostly a mounted lance-charger, but with some minor spellcasting. She's currently a L7 druid/fighter/hospitaler with L4 druid spellcasting, and her feats are Power Attack, Mounted Combat, Ride-By Attack, Spirited Charge, Natural Bond, Improved Bull Rush and Shocktrooper. I'm pretty sure that in 4e terms she's a striker (mobile, high damage to single targets).