A large portion of my dissertation revolved around the original C++0x concepts proposal. There are some amazing things you can do with concepts; I think the most important part of '0x concepts were 'concept_map's. However, I don't think they're anywhere near as important as truly lightweight coroutines, modules, etc. Mostly, they added a great deal of complexity to both the compiler implementation and, more importantly, day-to-day coding. One thing I don't see discussed is that if you don't want concepts without ugly & nonintuitive "holes" (backdoors), they tend to leak everywhere---especially into code you'd never expect to find them in.