I completely agree, the theme marketplace has become something of an arms race with everyone racing to have the most features. As a WordPress web design agency, we're constantly struggling with bloated, slow-to-load and difficult-to-maintain ThemeForest themes. Yes, themes need the flexibility to adapt to different people's content, but that doesn't mean that you have to pack in every feature under the sun at the expense of performance etc. I'd love to see more themes that go against this trend and combine flexibility with simplicity, performance, standards-compliance etc. And as you say, a major part of this is separating things out so that the theme is for the design, and the plugins are for the functionality.