OK, so I had a long talk with some friends of mine and this is more or less what we came up with as the most important part of dnd.
The key for any RPG is to be able to combine flexibility with specialization. Players need to be able to enact creative solutions that game designers (both makers and DMs) didn't think about, and each character needs to have some role that makes him useful to the party. A friend of mine mentioned that he made a healer in Star Wars 4.0 that was not really any more effective than a fighter that didn't try and specialize for healing at all. Builds need to mean something! Full disclosure: I never played 4.0, so my knowledge of it's shortcomings is limited.
I would like to see some kind of diminishing returns on power at high levels. Skills get pretty broken pretty fast in 3rd edition, e.g. escape artist checks letting you escape through a key hole. Without invoking magic, there needs to be a cap on what's possible, or player's stop THINKING and just start rolling, because the game mechanics let them.
Skills, Feats / Talents, Powers
The talent tree concept seems to be popular, but I don't think it stands as a full replacement for customizable feats and skills. Mainly, because we are limited to one of the trees that the game designers thought to put in. I really appreciate being able to make my character unique in ability as well as story. Even at level 3 in 3rd edition, there are dozens to hundreds of possible build choices for every class, some of which make more sense than others.
One thing I immensely enjoy about the skill system in 3rd edition is the ability to adjudicate complicated tactical ideas by clever combinations of skills and ability checks. I had a game once with a rogue whose primary combat weapon was a length of rope. The fact that that's possible is what makes DnD great. Absent that ability to use a player's ingenuity, we might as well be playing Skyrim.
The two big ideas here are Customization and Flexibility of Application. How you get there is largely personal taste, but something akin to a point buy system is necessary to some extent (in this respect, feats count as a type of point buy system...)
Magic Items are supplements to the game, not the core. A player with no magic items should still be useful. Maybe not AS useful, but still. I also like to see magic items with seemingly innocuous applications that can be used in coordinated ways. A player of mine once suffocated a kracken with a necklace of adaptation and some sovereign glue. Notice: players will hide WILDLY behind semantic arguments.
Magic Items are a fun reward that can give the players access to new abilities, which is always fun, but they shouldn't drive the mechanics of the game except as quest goals.
This is first point that our group has some disagreement about. We do want enough specialization that not everyone can do everything, and certainly not equally well. If you can do everything, what's the point of being challenged anymore? Players need to have a well-defined set of finite resources and capabilities, and if a challenge is best defeated by something outside that set, they must BE CREATIVE and work with what they have.
One of my player's had this to say: "I dont like pigeonholing characters into roles. My best and favorite character changed roles multiple times throughout the campaign (my Jedi master from the Star Wars game). 4th ed pretty much has it set up where each class is a specific role. and then if you dont have a balanced group contained one of each essential role then you get boned." I think that gets back to the importance of flexibility above. The rigidity of class roles can be assuaged through freedom in skill selection and feats in 3rd edition. As I understand it, the talent trees are a once-chosen, forever-chosen deal.
Now, that said, class roles are more or less the inevitable result of constraining player power. You can't be invincible AND kill things in one hit from the shadows AND dissolve walls AND remove debuffs and heal, you have to pick one. WoW makes that a pretty hard barrier, while 3rd edition DnD gives you a set of N points and says "put them where you want them" allowing for some interesting mix and match. I would like to see the mix-and-match style of choosing roles stay, but the abstract idea of differentiating players by party role will stay as long as characters are personalizable; there's nothing you can do about that.
The dreaded min/maxing. This will always be a temptation to players when customization is available, but I don't think it's the place of the RULESET to try and prevent. Stopping the players from become die-rolls instead of characters is the job of the DM and the other players around the table. There will always be that one guy determined to do 10d8 of damage by lvl 4 somehow, and sometimes that can be fun, but ultimately it's to the detriment of the gaming world. That's a balance that each table has to strike on its own. WotC should watch its wording, but other than that you guys should just offer the advice and let us figure out how to deal with it ourselves. You just have to make sure that the classes or abilities do not INHERENTLY break the world. There will always be combinations of abilities, especially for casters, that do this and escape your notice. Catch those you can, but we DM's are plenty capable of house-ruling fixes when a determined rules lawyer finds something you missed.