Writing simple code is also much harder then writing complicated code. If you write some complicated code at the limit of your mental capabilities you can not debug it, but you might also not be smart enough to write the simple code.

I guess this means that one should solve the appropriate problems for a given skill level

My #1 belief about engineering, and one I harp on constantly, is that simplicity is harder than complexity.

I use it as a heuristic. If my work is getting more complex, it's a warning sign that I might be doing something wrong or using the wrong approach. If it's getting simpler it means I might be headed in the right direction.