People are always drawn to the shiny new thing, but I have to admit that the changes to the combat mechanics might be the biggest draw to a new Vanilla version. I'm excited to play with ChiselNBits, as I have always loved what can be done with existing microblocks, but I've always felt they could be vastly improved. The update to TC will be fun to explore, as Azanor hasn't failed to impress me with his continued evolution.
But all is not without loss. Word on the street is Reika won't be updating any time soon due to life being life, so RotaryCraft, Reactorcraft, ElectriCraft, and ChromatiCraft (just to name the heavy hitters) won't be available for some time. I'm sure others will drop off as well, although many mods will be forked, passed off, or cloned. Seeing as my current pack is highly centered on the Reika suite, I will likely play it until a large stable pack is released.
If you're happy playing 1.6.4, play it. I had a ton of fun with it. But, as mod developers move on to new versions, support for the old becomes impossible, and I could only live with certain crashes and world corrupting bugs for so long before I packed up and moved to 1.7. Having a small private server of friends to keep happy and entertained was incentive as well, as some of my friends have attention spans rivaling a ferret on meth.
I have found FPS to be a bit lower on 1.7, but overall mod stability has increased substantially, in my experience. That said, a handful of mods have popped up to help optimize things (Fastcraft, BetterFPS, COFHTweaks, etc.). It takes some testing to see which combination helps given your particular computer and pack, but they can altar your experience in a pretty major way sometimes.
It's a big free world. Play what makes you happy, but know that the majority will be doing the next cool new thing (for better or worse).