Lauren Fernandez, in a comment to Stuart Foster about engaging blog lurkers, suggests people are either too busy or too lazy to take 5 minutes out of their day to add a comment to a blog post, even if it inspires them.
Do they comment? Sometimes. You have to leave it open ended, like you said. 250 words with holes that ask a lot of questions probably will drive more conversation that someone who is preaching for 700 words. I get bored after 300. If you want to hold my attention? It better be damn good.
Do her thoughts vibe with yours? Are you more apt to comment on short blog posts than long ones?
If you are willing to exercise your fingers in a comment below, I lend you my ears.