The villain. Every story has one of some kind. The evil mastermind forming a diabolical weapon that will destroy the universe. Sounds like an interesting villain, right? Well, he could be...but he may not be. Basically, it all comes down to his motive. If his motive is to get in the hero's way, it will get boring. Why does he want to get in the hero's way?
If we go back to the guy wanting to destroy the universe, he would want to get in the hero's way because the hero was trying to stop him from destroying the universe! That would be his motive. He also has to have a reason to desire the universe in smithereens. It could be that he wants to get back at somebody. It could be anything, just as long as it doesn't sound like he is doing it just because it makes a good novel! He has to have motive.
I strongly recommend that you check out Of Battles, Dragons, and Swords of Adamant, a blog by Gillian Adams for more tips on writing villains.