I just wanted to point out, one thing mod packs like Tekkit do is interlace mods. They don't just overlay them, they make them compatible. Tekkit has two different kinds of copper, but both can be used to make things from their respective mods interchangeably. That's why I like them, personally. Of course it isn't a very stable reason, but it's one thing I like since whenever I try to mod Minecraft something really weird snags and I give up.