What I like about 4e:
DMing, DMing, DMing, did I mention DMing? Have some more DMing while you're at it.
Rituals. Rituals could use some work, i'll admit, but over all I like them. Here are the spells that are cool, but don't belong in combat.
Healing Surges. I get it that HP doesn't entirely represent physical injury, but also the resolve to fight, thus allowing martial healing. It's not a difficult concept to grasp, I don't get why others have such a problem with it.
Item Rarity. People moaned that players got access to magic items in the PHB. Along comes rarity, a common sense approach to putting items back in the DM's hands. Implemented perfectly? Perhaps not, but it's a long way from players walking into a city and expecting awesome gear to jump off shelves at them.
Essentials. It's different, it challenges preconcieved concepts of the system, and allows for interesting takes on existing archetypes. I particularly like the hunter as a controller.
Skill Challenges. Reminded me of the system from Heroes of Battle for determining the outcome of battles. I loved that system, and skill challenges allows me to build scenarios in that vein. As a DM, I keep my skill challenges secret, letting players do what they want and I asking for the occaisional check to back up what they are doing, then recording the success or failure for the challenge. I'll also have skill challenges spread out over multiple encounters or longer.
Choice of stats for defenses.
No hit dice. I don't want to be a glass barbarian with tons of strength, rolling 1s and 2s for HP each level. I'm perfectly content gaining a set amount of xp every level. I also don't want to be a 1 hit and I'm dead wizard.
Removal of blunt/piercing/whatever damage type silliness. Don't care for it.
What I don't like about 4e:
Formerly busted math. I don't see this as much of an issue anymore, other than having to manually convert older monsters, and it looks like even that is starting to get addressed.
Skill challenges. This is more due to DMs dealing with time constraints during public games such as LFR that skill challenges turn from a behind the scenes mechanic to the AMTRAK station, with the DM going around the table asking each player what they want to do.
Item Rarity. There needs to be a larger number of low level items, esp miscellaneous items that are useful, but aren't part of combat, typically. I'm talking things like feather fall tokens and such. Eberron has a vibrant low level magic item economy? Hard to tell based on existing items. Again, I can see the logic in this, players want magic items that allow them to more effectively kill things and take their stuff, how much room do we give items that don't do this? Lets give some examples and leave the rest for someone else or the DM.
Rituals/Spells in general. I believe there is a space between rituals and combat spells that is currently missing in 4e magic. I'd like that gap to be filled, but I don't know by what. Obviously there can be some balance issues here and it's a good place for a game to break.
DDI. It's getting better, very very slowly. Hopefully that will change with the VTT and a better release of the monster slider. I'm really disappointed from what was promised at launch that never come to fruition. I'm also displeased by Dra and Dun no longer being bundled. I also miss having them in print, but I can do without the many pages of ads from back in the print days. I just print them myself now, and store in binders.
Artificer. They just don't have the feel from 3.x anymore. The class needs a reboot.
What I like about other D20 systems (mostly PF):
PF Goblins. Not the same as my Eberron goblins, but damn if they aren't cute as a button made from a rusty razorblade!
PF Artwork. It's pretty.
Alchemist and Witch from PF APG. These classes look damned awesome. I'd love to see some of the stuff from alchemist moved into a 4e artificer.
Paizo's adventures. Fantastic adventures!
Saga Edition. Much love there. Sad to see it go, but i think they covered just about everything by the end. Particularly liked the injury track thing they had.
Eberron's 3.x cosmology.
What I don't like about other D20 systems:
3rd party content glut. Sure, the OGL has allowed some real gems to come to fruition, but it's surrounded by a garbage dump of utter crap that makes one want to avoid anything 3rd party. This has not changed with PF.
Perform ___/Craft ____ skills. I'm a hero, why am I bothering to craft arrows. They are straight sticks with feathers on them. This could be hand waived as something my chatacter just does bc he lives in the woods. Why am I crafting a sword? I pay sweaty ppl to spend months forging my blade from the star rock i found in the dragons' layer after I killed it. Then I pay someone else to turn the dragon's hide into my nifty new armor while i'm off fighting more monsters and sampling tavern wine and wenches. Craft (basketweaving) is never going to be useful to me. Anything I crafted was before I became an adventurer. Adventurering is where the real money is.
Perform... well.. I guess... but just make it freaking Perform, one skill, not a seperate skill for every damned instrument. If I take perform, I should select a group of instruments like weapon proficiencies. Take feats to be able to use exotic instruments. Simple woodwinds, simple strings, simple percussion, simple brass, complex woodwind, string, percussion, brass, exotic instruments. Pick a couple groups at creation, based on some stat mod math, and get more as feats. Done! Now I can rock out on my Astral Banjo of Dueling. I can also write some adventures that may require knowledge in performing an instrument. Since I know there are groupings of instruments, I don't have to worry too much about if Elvii Priestly the Bard can play a double guitar, he's got complex string instrument proficiency.
CRs. Don't like them. I like a battle with more than one or two enemies versus the party. I like knowing that this given monster should be a challenge to a given player of same level, and that x monsters is a challenge of x players. 4e got this one right.
Martial classes. 3.x gives spell casters lots of love. Good for them. Fighters want a piece of the action too. I enjoyed, what was it, Bo9S?, that brought new life into martial classes.