The story was absolute crap, but the campaign levels were still really fun.
Also, each campaign did feel like a full fledged game from a content perspective. I can give blizzard shit for a lot, but how they handled sc2 (beyond dropping it completely) is low on the list.
I remember reading somewhere that Below zero was originally intended to be an expansion, but got changed into a standalone release. The subnautica 2 they are working on now is entirely new.