I believe the implied meaning is:

+0 -> support from the sidelines (like for a change that doesn't directly affect you)/something that's not urgent but would be nice

-0 -> disagree from the sidelines/disagree with the solution but can't think of a better one

I would enjoy it if they used +ε and -ε for that, although +0 and -0 is probably clearer.

like that idea! Dunno how many people have epsilon easily accessible on their keyboards though.

Those terms for code-review voting are (almost certainly) from Apache, which treats them differently. From http://www.apache.org/foundation/voting.html :

+0: 'I don't feel strongly about it, but I'm okay with this.'

-0: 'I won't get in the way, but I'd rather we didn't do this.'

(These votes are strings, not numbers, though they closely resemble numbers. See also the existence of a "++1" vote.)

Arithmetically true, but they have different IEEE 754 floating point representations.

Also, my inner mathematician understands that function limits can tend towards zero from above, or from below.

Zero ain't always just a big fat zero.