Replayed Dark Messiah not so long time ago it was a really good game. Went to recheck the reviews too, only after I've finished it.
Most of the critic, bad scores was because of it's performance (new graphics cards were unaffordable by the masses) and technical difficulties and the fact that it completely left out LAN and barely anyone ever got the experience the multiplayer because it was one of the first adopters of Steam when Steam still sucked.
It was way ahead of it's time, and that counted against it. Also was released around the same time as oblivion and due to it's modding capabilities remained in people's mind a long time.