I find that every new site that I write for expands my skills. Years ago, I started with eHow, then wrote for Helium, List My Five, Squidoo, Bubblews and now for Hubpages, Webnuggetz, Daily Two Cents and Listofied.
Hubpages requires a focused topic, where Squidoo encouraged broader pages. I had some really long pages on Squidoo that didn't transfer well into Hubs. By narrowing the focus, these are now featured again.