I voted for Action Points, Critical Hits, Feats, Healing Surges, Kits, Magic Missiles Never Miss, Non-vancian magic, Prestige Classes, Skills, and Weapon Versus Armor Table.
Action Points: I like a player being able to somewhat negate the randomness a d20 institutes when they feel an important outcome rests in their hands, and adding some form of bonus seems to be the best way to do this.
Critical Hits: Who doesn't like smacking the goblin just that much harder when your die shows up a natural 20?
Feats: Though I felt a lot was just bloat, I like being able to tweak my character to shape them into what I want them to be after level 1.
Healing Surges: I like the idea that there is a limited amount of stuff any one person's body can go through in a day. I thought the implementation in 4e could have used some work, but with a little polish I think Healing Surges make the abstraction of HP work that much more and also eliminate that "quiver of CLW wands" mentality.
Kits: I like saying "I'm not a thief-y rogue, I'm a dashing swashbuckler" and having more than fluff to represent that, no matter what mechanic lets me do it. And I want this to occur at level 1.
Magic Missiles Never Miss: Its just cool to have automatic damage when you can't throw fireballs anymore.
Non-Vancian Magic: Certain parts of Spells per Day feel artificial to me, such as not being able to "forget" your 3rd use of Fireball to use some other spell you realize now would help a lot more in the coming encounters. Though I would be fine with Vancian magic if they provided default explanations of why spells work this way.
Prestige Classes: It adds a sense of advancement and scale to the game world, imo. But, that being said, I think they should be limited in number and each should provide unique mechanics and flavor: making your character cooler, not more powerful than core classes.
Skills: We aren't defined by our innate attributes completely, and have the ability to learn things even if we are not very talented in that area. I, for example, would not consider myself a very charismatic person, but I can play the saxophone quite well. Characters should be able to become decent at things their attributes are opposed to as well. This should be a module imo though.
Weapons Versus Armor Table: Plate sucks versus crossbows, chain doesn't help versus hammers, etc. These are why so many types of armor and weapons exist in real life so I would like a module to add this layer of simulation to my game.