- low effort for drive-by downmods (there are stupid numbers of comments blasted to nothing despite being insightful or informative)
- upvote doesn't distinguish between 'I agree' and 'hey, this is really informative'; the former is mostly meaningless, the latter is useful
- single downmod diminishes what you said and makes it harder to read (on an already low-contrast site). Hardly fair given the ease of 'drive-bys'.
The forum I spend most of my time on has a reason required for the mod - it's not this brainless +/- system that the web is moving towards. You get an idea why people are up/downmodding, and it raises the bar ever so slightly so that it takes actual effort to mod (that is, critique) a comment. The displayed point value is also capped - while the user gets the karma for the mod, it's not displayed beyond a certain cap.
Mostly I see the benefit of this system as stopping mindless up/downvoting - requiring the tiniest bit of effort produces better quality results. Still not perfect, but much, much better.