Unfortunately this idea isn't going to increase the quality of commenting, it is just going to encourage stupid people to pad out their nonsense, Steve Yegge style. It's also going to punish people whose witty remarks are actually witty.
I think that one of the things that holds people to very high standards here is their respect for the community and the many members that have something interesting or insightful to say.
I have (almost) never experienced a comment that was an obvious digg-user unwittingly passing by, but many comments from new users that want to join the conversation but haven't been here long enough to get a feel for the unwritten rules, and thus post comments that may not go well with the rest of the community.
These users will definitely be helped by a small reminder that they should contribute in a meaningful way. The challenge, as you hint at, is of course what this reminder should be.
Maybe something short and witty like "Remember: if it isn't worht reading it isn't worth posting"
"Remember: if it isn't worht reading it isn't worth posting"
Most people think everything they have to say is worth posting. I'd rather something specifically aimed at short comments, as mentioned above. Something harsh aimed at something under, say, 100 characters, and a brief little reminder towards something under 100 words?
I wouldn't prevent people from posting short comments, just ask "are you sure?" I don't think anyone would go to the trouble of making their comments longer just to avoid an "are you sure?" screen.
I hardly ever read EULA's (which this in some ways is), and I'm probably not alone in this behaviour.