So, I am using the TechnicPack pack when I play Minecraft (not much these days, because Diablo 3), and it is a true gamechanger. One very good example of why mods make Minecraft into so much more than it is initially is this.
A nuclear reactor designed by an electrical engineer. Sure, you can build a nuclear reactor without much trouble with the mods in that pack, but making it in such a way that it mirrors the real world counterpart? Not as easy. This guy nailed it it seems.
So duders, go install this pack, build crazy things that work and showcase it here.