This thread on RPGnet contains a discussion on why people still play early editions of D&D instead of 4e. Some of the options presented by the original poster seem to miss the point, but the debate looks healthy and energetic.
As for myself, I run old-school D&D because it’s clean and simple and flexible, requiring little prep and allowing me to wing it without feeling like I’m breaking the rules. The thought of writing up 3e stat blocks again makes my brain bleed, and 4e looks far too rigid in format and playstyle for my taste.
What about you? Why do you play old-school D&D instead of the newer editions?