One of the biggest problems I've found for me is forcing techniques. In other words, if it doesn't work the first time, push it harder and make it work. The drawback is that the mechanics of my techniques aren't nearly as smooth as they could be. Then I get an uke who has more muscle mass, and all of a sudden, my techniques don't work.
I think strength and conditioning are important, but at the beginning when you are first learning, you are better off not being too strong.