WordPress plugins are great, they really are. One problem with them is they often include their own styles and scripts. The problem here is that several plugins with useful features mean a browser needs to download 10+ javascripts and stylesheets. This isn't good for page load. It's awful. See:
http://www.die.net/musings/page_load_time/
My suggestion would be an API that allows all plugin/css to be included in 1 PHP generated CSS and Javascript. This would drastically consolidate requests. For performance reasons it should ideally be cached so that it's only regenerated when a plugin is loaded/reloaded.
This is becoming a bigger issue as plugins become more common and useful. I hope a solution is found that gives plugin authors the freedom they need, and blog owners the performance they want, without having to sacrifice features. I think channeling all the requests into 1 JS and 1 CSS file would achieve that. They could use a numerical ranking system to calculate position in the file (to avoid conflicts) similar to how (iirc) filter work.