I don't really miss much from AD&D, although every now and again I get the urge to run a light-hearted session using all the fiddly bits from AD&D, including Bend Bars/Lift Gates checks and other stuff that never really came up in my sessions.
I do miss some aspects from classic D&D, though. Particularly the clear division between stages in an adventurer's career. I liked the emphasis on the progression of an adventurer as the game moved from dungeon crawling (basic) to wilderness exploration (expert) to rulership (companion) to world-hopping (master) to godhood itself (immortal). Most of all, I miss the Rules Cyclopedia, because having just one rulebook needed to run D&D was awesome.
That said, most of what I miss about older editions can be used in 3e. The progression of adventuring styles is easy enough to manufacture in most games, although it would be nice to see more support for non-dungeon crawling adventures from WotC. The one rulebook thing is harder to manage, but can be somewhat achieved by using select printings from the SRD to complement the Player's Handbook.