I personally reserve -2 for "this is a
fundamentally bad idea" or "this requires
community consensus before being implemented". Anything that is fixable in the
code should get a -1 or 0.
Btw, I personally prefer to get -1 reviews over 0 reviews, simply because it's
easier to spot them as "todo" on the gerrit dashboard. If gerrit would
"stuff with new comments" more prominently, I'd probably use 0 more often.
I treat -1 as "this needs to be fixed before it can go in, but once it
is fixed it's good". Agree on -2. I use 0 for just commenting on things
where I do not feel qualified or entitled to review things but still
have something to say, like additional todo items or general discussion.
So, most reviews should be +1/-1.