Oh for the love of Notch...
What is causing all the problems with updating mods to 1.6.x? Is a back-end change that fundamentally changes the way parts of mods are coded. It is a MUCH faster and easier thing to do now, but that doesn't mean it is either fast or easy, just that it is a significant improvement over how it used to be. This means fewer system resources used, which means more FPS and able to run on crappier computers easier. This also means future mod updates are going to be a lot faster. However, for right now? It means re-writing several thousand lines of code. A one-time hassle, which pays of BIG dividends later.
Part of it also has to do with the new Resource Packs instead of just Texture Packs. That means that these Resource Packs can include new custom sounds as well, making your Minecrafting experience even MORE customizable. Don't like the sound from x mod? You can change that. However, it also changed how textures work, which is causing several mod authors to have to re-write significant portions of their code. Again, one-time hassle, which pays big dividends later.
Some people might wish to be impatient and demand results now. For myself? I'm all for these changes.
Speaking as a mod pack compiler, the incompatibility with previous worlds means I get to go in and tweak ID settings without pissing off my entire user-base, since the worlds are already incompatible. This lets me do a few things for my users that I didn't think about when I first created the mod pack, but was essentially stuck with because it would require changing ID's which would boop up existing worlds. This means I can set up the mod pack in such a way that adding in additional mods would be exceedingly user-friendly. This gives users unprecedented ease of adding additional mods for my mod pack. Since my mod pack is a 'Minimalist' mod pack, with only a few 'key' mods, suitable for people new to modded minecraft or those who simply don't want to deal with a flood of mods, being able to 'bolt on' additional mods yourself would give it additional use as a 'core' mod pack to tweak.
I've also got another idea for my mod pack which might present an entire paradigm change, much to the advantage of the end-user, but I'm still looking into this and not ready to release what it might be. Yet. Suffice to say, KirinDave pointed me at something that might well be a game-changer for mod packs, on the back-end, and I'm really excited to see where it is going to end up. Which, again, I would never have been able to do for my users without introducing world-incompatibility. So the fact that 1.6.x already does this means I can get everything down in one pill, so to speak, and provide a whole new level of enjoyment and entertainment for my user base.
Yes, being unable to import your 1.4.x or 1.5.x worlds into 1.6.x means you'll have to do a one-time restart. However, this gives me the freedom to do things for my users which would also require a restart, and get it all done at once, so you will only have a single restart necessary rather than a shotgun of several restarts. Otherwise, I would have had a hard choice of forcing my users to do a reset anyways but getting all these additional features, or simply holding my peace and not including these features. I think it will very much be worth the restart, though.