I'm very new here, but I'll say what I think anyway.
I think we need to be explicit about partial code reviews being ok. I think we ought to start putting things like "This is a partial review focusing on good variable names" at the top of answers. That way, people who are new to the site see partial reviews being done and don't have to worry about (or ask) whether that sort of thing is ok around here.
Same thing with upvotes. I haven't been here long, and I've felt like there aren't enough votes around here, but looking at my stats, I've only upvoted twice. I've been on Programmers longer, but I've got 201 votes over there, and it feels like a smaller thing to upvote there than here. An upvote shouldn't feel like a big deal. It should seem like the sort of thing people do around here, not an unusual thing for special circumstances.
Let me put it this way -- if I went through all of the questions on the front page, and upvoted all of them, the number of upvotes on the front page would more than double.
As far as the backlog goes, I think that's a bit of an artificial problem. The OP's of the 2 year old unanswered questions are long gone. They aren't there to edit their questions so the questions aren't the code equivalent of a wall of text, or to clarify anything. They aren't going to be there to learn anything, and they might not need to learn from the problems in their old code because they've already learned about it from elsewhere.
It would be worth reviewing code even when the OP almost certainly won't see the review if it were probable that someone else might read the review. If I'm pretty sure that reviewing an old unanswered question will result in me spending an hour of my time writing something that nobody else will read, I'm not likely to do it. I'd rather get a downvote and an explanation of why I'm dumb than have nobody read an answer.
I think worrying about not letting recent unanswered questions slip through the cracks would be useful, but I see a lot less benefit in worrying about things that have long since slipped through the cracks. On a lot of stackexchange sites, answering a question long after the OP has vanished makes sense, because other people are likely to want to ask the same question and benefit from a good answer. I don't think it makes as much sense here.
If reviewers could be reasonably sure of getting an upvote or two (or maybe even a downvote) for a review that doesn't cover everything, they'd have a decent motive to spend time on a question even when the OP is long gone.
EDIT: I've looked at a couple of old/unanswered questions and there's something I've seen a few times that I think is pretty common: people answering in the comments. This question for example.This question for example. The post has no answers, but it has helpful comments, and all of the helpful comments (which the OP noted and appreciated) were within 32 minutes of the question being posted. By the numbers, we haven't answered this question at all, but in reality, it was answered helpfully less than an hour after it was posted.
How should we handle questions like this? Re-answer them for no good reason, hoping to get our numbers up? Leave them alone? Close them?