One of the problems most people face when thinking about playing D&D in ME is the magic system. Even Tolkien scholars can't agree about ME's magic.
The good part for us players and DMs who wish to play in the 3rd Age of ME is that we don't need to care about that. And now with 4E, we need to care even less.
The main characters are not magic-users (arcane and divine), they are all Martial characters. When you accept that and just get past the magic problem, everything else is very easy to adapt. We have an even greater number of pure martial classes now in 4E: Fighters, rogues, rangers(spelless!) and warlords. With these 4 classes combined we have 99% of 3AME characters covered.
If you need some characters with little magical abilites, from their special bloodlines, elven nature, blessings of the valar, whatever, just allow them access to it via feats and powers. Some elves can sense powerful creatures, but they don't need to have a level in Wizard class in order to gain Detect Magic, all they need is access to the elven feat Sense Power. If your noble numenorean character has the ability to heal people with his bare hands, the character just get the Class Trainning feat to gain Lay on Hands Power.
Sense Power (elf)
Tier: Heroic
Requirements: Must be elf or half-elf.
Benefit: You sense the presence of any creature from the Epic tier within 100 squares.
All truly magic-users are NPCs anyway, and with the new NPC creating philoshophy, it's much easier to design Gandalf without having to worrying if he is a Wizard or Cleric or Paladin. Just give him some Gandalf-ish powers, lots of HPs, good attack and defenses, a magic sword, and you are ready to go. His class is just Gandalf.
You Shall Not Pass
Daily+Arcane, Implement
Standard Action Close burst 3
Target: One creature
Attack: Charisma vs. Will
Hit: The target is immobilized (save ends).
Miss: The target is slowed (save ends).
Magic Items in ME are also very rare, but the designers already told us it will be easier to get rid of them in 4E. All you need is to adjust the character's Attack and Defense bonuses and some other minor adjustments. It's easier to introduce more flavorful magic items without having to worry about the increase the character's power. It's also easier to understand how ME elves create their "magical" tools. They have access to elven crafting rituals. The same with dwarves and their dwarven pseudo-magical things.
I just can't wait to get 4E and start adapting it to Middle-earth.