Keeping it positive, and speaking from what I have played:
Moldvay/Cook B/X D&D. ReadySetGO! Fast chargen. You play now! Get a good DM and you can just go, with little need for understanding what is going on. It hooked me in as a kid and I would gladly play it today.
1st ed AD&D. DM's Tinkertoy. It greatly encouraged houserules, because it had so many little subsystems for so many things. A good DM would make this puppy sing!
2nd ed AD&D. A Whole New World(s)...A variety of interesting and well-described settings. Showed that you could go well beyond the normal pseudo-medieval. I think this one encouraged DM world-building too (partly by example, and partly by DM's wanting to put cool feature from setting A into setting B), in the way that 1st ed encouraged DM house-ruling.
3e D&D. The Referee. A lot more codification of the rules. Now you can have a fair and good game even with a relatively inexperienced DM. A plethora of codified choices. Encouraged "shopping for super-powers" to quote someone who I have forgotten the name of (Robin Laws, maybe?). This hooked a lot of players and DMs and helps those that like customization.
4th ed. The Game's the Thing. Wayyyy more focused on the pulse-pounding battle itself! Encourages group tactics. Best DM advice book D&D has yet produced.
The negatives can be handled by a competant DM. Some are merely the lack of certain good features in other games. 3rd ed char gen just takes longer than B/X chargen and for those that want faster chargen that will be an issue. Some of the options of 3rd ed chargen are not group-friendly (Frenzied Berserker gets autobanned in all my 3e games). 1st and 2nd ed. was hard on non-rogue demi-humans at high levels, and 3rd ed was hard for some DMs to run at high levels. B/X D&D offered less mechanical choices unless the DM was willing to get mechanically creative. Psionics really didn't get balanced until 3.5, and don't exist yet in 4e. I imagine 4th ed will have some issues too, though it is too new for me to really say yet. Mainly the 4e beef I hear is that some popular choices are not there yet, to which I counsel...patience!
