Dedekind said:
Certainly that is an issue. But also don't forget that though crunch will not translate, the soft stuff will. The FR source books will still be useful (with some exceptions) as will Eberron. For that matter, Ptolus and all it's detail will be useful even if the rules are different.
So, if we can agree that fluff translates but crunch doesn't translate, what is in these books that you want so much that is not in 4e already? That isn't meant to be snarky; I really want to know what from non-core 3E rules you want in 4e that will not be delivered. If your game is already 4E-like because of these extra rules, is there not a benefit to converting to a system where the "options come standard"?
Actually, the very top of the list is adventures. I love published adventures because I don't have as much time as I would like to prep my own. And I have a lot of great adventures that I want to use (currently running Expedition to Ravenloft, and hoping to start Curse of the Crimson Throne by summer, plus stacks of GameMastery Modules, Demonweb Pits, Eyes of the Lich Queen, heck, I'm still hoping to run Savage Tide someday). All of these *could* be converted to 4e, but like I said, I don't have the prep time I'd like, so I don't want to take the time to convert. That's the biggest reason.
Secondly would be the alternate classes. I'm notorious for playing odd characters. At least 1/3 of all my PCs have been psionic but there won't be 4e psionic rules for at least a year (or two if they can't fit it in with primal and the extra arcane and divine). There's always faking it with a wizard, but why fake it when I can still use the 3.5 psionic rules and the extra material Dreamscarred is publishing? Also Tome of Magic is probably my favorite 3.x book out there. Although Incarnum and Book of 9 Swords were a few notches lower, they are the kinds of books I like. The ones that play with the rules and experiment with new systems. So it's not just the fluff, but I really like the crunch of those books. One of the things that is turning me off from 4e is the fact that all the classes basically resource manage the same. After the incredible variety from late 3.5, having them all function the same with just different power choices just doesn't seem as interesting to me. I won't deny it's an order of magnitude more balanced, but that hasn't been as much of an issue for me. Classes that resource manage different can feel different to play.
Don't worry, you didn't come off snarky to me, and I hope I'm not sounding snarky either. I have no doubt that 4e is a great game and massively more balanced. Just when looking at the two side by side (well, to be fair, the 4e previews and my shelves of 3.5), 3.5 just looks like more fun to me. But there's plenty from 4e I like and does appear to be great fixes like the monster and NPC creation rules. Although that's more of a philosophy and set rules so far and can be retrofitted to 3.5 if you don't mind a "goblin shaman" being a 5HD humanoid with a handful of random spell-like abilities. I do plan on using that mindset for monster and NPC creation rather than the NPCs following PC creation rules mindset of 3.5.
However, I'll also fess up that as a part time freelance writer, I'll certainly take the time to learn 4e and try writing for it (in fact, I've paid money into Open Design hoping I could have seen the rules months ago, but ah well). However, for this discussion I'm primarily talking about my home games rather than playtesting writing and such.
On top of it, the group I game with the most took a couple of years to finally move from 3.0 to 3.5, so none of them are really interested in getting new books any time soon. So it might be kinda moot what my opinion is anyway. In fact, several of them who only buy the PHB or at most the core set are outright pissed that PHB 1 and MM 1 won't have all of the standard stuff and certainly don't want to shell out money every year for another core set. I think that's pretty cheap for a game they can spend so much time playing, but that's their opinion. WotC doesn't get much money from them anyway, but WotC might lose some of my money (and I've been a self-described WotC whore) because they pissed off the rest of my group. *shrug*
Sorry for the long-windedness, but you asked.

For all those various reasons above, I'm not real interested in converting to 4e any day soon, even if it is a better designed, faster playing game. But I was on the fence (or at least figuring I'd convert after PHB 2 or something) until I started to see more info from D&DXP as well as talked to the rest of my group about it to get their feelings. As a freelancer, I'll probably get the core set and try it out or at least try DDI for a while to stay on top of 4e (especially if they ever release a decent GSL or go back to the OGL), but I might not eagerly crave WotC's latest releases quite like I did before. Or, who knows, there may be some magic in 4e that hasn't been previewed that lets me do the sorts of stuff that I want. *shrug* All I know is that my main group has made up its mind to keep playing 3.5 for at least another 2-3 years (we're about to start another very long campaign they definitely don't want to switch to 4e now for reasons above).