There were quite a few complaints in Open Beta by people who had just found the game too easy.
I am not necessarily one of those people, yet have found a way to increase the difficulty.
So, if you are one of those people who have found a game exploit, a character build who is so maximized that you cannot fail, or you are just that darn good - feel free to do this rather than complain that the DEVS did not make the game to your elite level of play.
Simply team up with one or more people. Find a group and team up. Set loot to free-for-all. Just do not actually do the missions with eachother. The more people you group with, the harder the game will be. You will still be able to loot everything that drops. There will just be more enemies, and sometimes even more powerful enemies.
Congratulations, you have now used innovative thinking to match innovative thinking. Your uber skill/build will now be tested to see just how good it is.
I hope this helps anyone looking for more challenge out of this game.
**DISCLAIMER** This is not meant to be a flame post, nor do I suggest you actually do this. However the option is there for people looking for a little extra challenge.