Well vanilla minecraft will run on most people's computer perfectly fine. It's modded minecraft that will cause problems to people's computers. So unless Jeb wants to think modding is an intergral part of Minecraft and optimize accordingly, I don't think Mojang will bother optimizing minecraft anytime soon. (well until the modding API comes out)
What minecraft do you play that has 100 plus mods or is it custom
it is custom. But most Feed The Beast packs have 100 plus mods.
I use the Direwolf20 pack which has 140 mods and I use a 128x texture pack which includes most of those mods, I get around 60 fps with 16gb or ram, 17 3770k, and a gtx 770. It's the most demanding game I've ever played.