I like Google Reader's option to mark articles with a star to be revisited later, and the ease of adding new feeds in the full client; from Bloglines, I like the fact that it puts all new articles in one page - minimal UI interaction, and I don't need to keep a connection up.

One area I'm hoping to improve on both is the speed - both Bloglines and Google Reader, on the Blackberry at least, are a bit cumbersome and need several clicks before you actually get to read anything. Another thing I'd like to do better is that (with Bloglines) a failed page load still marks all new articles as 'read' - this does happen occasionally when I'm on the move, and I end up missing the latest articles.

I have one or two other ideas to experiment with :-)