I feel exploring a mod and all it's possibilities to a max is actually preety good.
Forestry's and IC2's bees/crops system is preety cool.
I like being able to just relax and check "Hey, what's that thing I've spent ours trying to get and it was relaxing doing?" or "Bees! What do I have in this one?"
TC3 research system is cool. Requiring tons of aspects to craft anything past a point where it will be REALLY useful?
Nope.
Not cool.