At the moment, MathOverflow has tag lower-bounds (100 questions), but no tag upper-bounds. Are lower bounds that much harder to compute? Or that much more interesting?
Worse, due to the non-existence of tag upper-bounds, there are questions on finding upper bounds with the tag lower-bounds, e.g., Good upper bound for a certain sum.
Hence, I propose that tag upper-bounds be created or that tag lower-bounds be renamed — say, upper-lower-bounds, like on Mathematics SE, or simply bounds.