First off, let me just state that I prefer 3rd to previous editions, severely. Although there are more than a few things I can think of.
-Most importantly, the quantity of the books. The monster manuals from second edition were about the size of bibles. Although part of me likes collecting D&D books, other parts of me hate buying expensive books just to complete my collection of monsters, and having all of the monsters scattered around in various sources. Also look at the content of 1st edition deities and demi-gods vs. 3rd edition. There's no contest.
-Monsters from folklore that are ignored in 3rd for bizarre reasons. Creature Catalogue helps a lot, but I still don't get why Leprechauns, Brownies, and Poltergeists aren't in the Monster Manual
-The TSR logo
-The minimalist look of the 1st edition illustrations. Although I'm a bigger fan of 3rd edition illustrations than most.
-Actual good modules (Although a third edition module invented the lovable Meepo)
-Actual existent modules
-Even though books contained more monsters than they do in 3rd edition, each monster had an ecology and society section. Also, I miss the more Euclidian organization of the monster manuals. 3.5 was a step back (I miss the yellow block stats), although apparently they're reviving the old method from MMIII.
-New editions not coming out every three years and not containing fractions
-Cool settings, like Spelljammer and Al-Quadeem. (I probably misspelled that)
-WOTC, or the absence of, rather.
Yeah, I know, minor things. My list of things I'm glad are gone from earlier editions would be much better