What game mechanics do you just love?
I really enjoy Feats. I also naturally enjoy the Fighter class and its Bonus Feat list. I love the way the same simple class can represent a Knight, an Archer, a Pikeman, a Hoplite, a Centurion, etc. In fact, I wish all the classes used Bonus Feats rather than set lists of Special Abilities. (There's no reason Bonus Feats have to be combat Feats, after all.)
Are there any mechanics from earlier editions that you miss?
Not that I can think of.
What mechanics from other games do you really like? Can you retrofit them into D&D?
From other d20 games, I love the Defense bonus, elements of the Wound/Vitality system (mainly just starting with more Hit Points), armor as DR, and Call of Cthulhu's magic with a cost. Those are all easy to retrofit into D&D, except the last one; you have to replace many of the classes (and you probably want to scale things a bit differently, so everyone isn't going insane at the drop of a hat).
From Pendragon, I like the Trait system, and I've suggested using Law/Chaos and Good/Evil as a simple Trait system for D&D (particularly for Paladins and Clerics). I also like it's notion of a Winter Phase or "off season" between adventures, with plenty of mini-scenarios to play out back in town or back at the castle.
From Hero/Champions, I've always marveled at the line drawn between game mechanics and "special effects". You can use the same rules for an ice blast, fire blast, optic blast, etc. A guy who can run up walls and over water really just has a limited form of flight. You can even fit this into D&D a bit, most obviously by modifying existing spells with cool "flavor", or by reinterpreting existing feats slightly.