Update: Of course, my feed doesn’t validate either. Two of the three problems are simple fixes, the third will take a little massaging to get right.
I’ve corresponded privately with a few folks whose feeds break in my aggregator (I use NewsGator) — and this is a huge problem. Surprisingly, many of the commercial blogging apps generate invalid feeds out of the box — and most users have no clue what to do about it (indeed, they rarely know there’s a problem). Step one: subscribe to your own feed. Step two: throw your feed through the Feed Validator to see if anything stands out. If there are problems, you could very well be making it harder (if not impossible) for others to read your site.