All the stuff that DM Jeff cites and about half the stuff Celebrim cites* are reasons for me. Especially the bit about WotC telling me things suck about 4e that I don't happen to think suck. There are some changes that sound really positive to me (no more full attacks! Touch AC = reflex DC!) The problem is that for everything I see them doing that seems a bona fide improvement, I hear about another change that seems to be ill-justified and/or a step backwards from my perspective.
* - With the particular exception that I always liked Mearls' work before he went to work for WotC. But when I saw the article about the "kinder gentler rust monster", I knew there was trouble ahead.
However, the principal reason is somewhat in line with what Spoony alludes to here:
Michael Morris said:
The 3e design team had one major advantage over the 4e one - they had a guy that had been there since near to the game's beginning - Skip Williams. Now this is a guess - but I'm thinking Skip helped keep Monte and Bruce in check with some of the wilder changes that were proposed and dismissed (and now lost to the mists of time).
I don't know if I'd lay all the responsibility on Skip here, but I do feel that when entertaining changes, you need to keep touch with why some people play the game through various editions.
I may frequently disagree with changes that the design team are making, but when it comes down to it, most of them are experienced, intelligent designers and will produce a playable, fun game. But I see a shift in the nature of the background material and concepts that have grown up from 1e-3e. ("Fluff" if you will, but more "metasetting".) So it may be a great game, it will be a game like Exalted or Burning Wheel that might be an interesting fun game, but no longer a game encapsulating the core concepts that are part of the continuing campaign I have had since 1e. As such, like those games, they have to "start fresh" with earning my interest.
I want continued support of classic creatures, classes, races, planes,
as they existed and evolved over 1e-3e etc:
1) Because I like them.
2) As Erik Mona put it, it's part of the imaginative framework I am used to using. When I think of adventure for my game, I think in terms of things I have learned over the years. To have ideas I want to use shot down or too much of a PITA to implement is not something I am willing to put up with. If it's more work to run the adventures I want to run in a new edition than the old, then the old edition is the logical choice for me to use.
3) At no time have the ideas of the current spate of designers been so good and so complete that the idea of earlier designers (editions) not been worth plugging into. So long as the same basic assumptions exist in the game, I can continue to tap into previous editions' material; I have been able to do so easily for all editions up to and including third edition. But 4th edition proposes a shift in the baseline cosmology, races, creatures, and classes significant enough that this usage of older material will be compromised.
Ultimately, I'll probably play 4e just like I play a variety of other non-D&D games. But I don't see 4e replacing 3e as my edition of choice for my home games.