I don't think "should have a score of +2" should enter into the decision to upvote, downvote, or not vote.

Why not? No, really: what's wrong with that?

Instead, I'd rather voting algorithms which, when implemented individually, have results which can be meaningfully summed.

The current voting algorithms can be meaningfully summed, they're just complicated, opaque and nonstandardized. I don't understand why you think "everyone should use my voting algorithm" is a useful thing to say.

If I think a comment should end up with a score of +2, too bad, I have no direct way of controlling that.

In what situation would you not, given that it is possible to alter your voting decision based on whether the score gets closer to +2? Do you intend to prevent that somehow?

do people downvote comments that they would have otherwise not voted on if they think the comment has too many upvotes?

At least two people do. Why do you ask? (Seriously, I can't figure out why this is phrased as a rhetorical question.)

Edit: Okay, here's the thing: I think it would be more useful if karma was the average of our valuations; i.e. if you could, say, input '+10' or '-3' as shorthand for 'upvote if below this number, downvote if above' rather than simply 'upvote' and 'downvote'. What do you imagine the problem with this system would be?