Diablo 3 was has that got to do with anything.
More like reviwers comparing it to the likes of Oblivion...
Guess you do not read to many reviews or PC game magazines, because most reviewers who gave Divinity 2 a low score have mentioned Diablo 3, and how they know it will be worth the wait, thus they have already given their opinion on a game that has not even been released yet. ;-)
Which review(s) are you referring to? I just read the bottom 10 or so (worst) reviews from metacritic and did not see Diablo 3 mentioned once. I saw one comment saying the divinity series is "diablo-like" but that's about.
Even if there is one review that does mention it...that's hardly "most". It seems this game was reviewed low by "most" reviewers on Divinity 2's own quality (or lack of).
Personally I am really enjoying the game (about 30 hours in) but it has many rough spots so a lot of the negative points brought up in reviews I think have merit.