In what ways did you find the level design to be lacking, having never played the game?
It hasn't even gotten "great" reviews, it has gotten "good" reviews on average. It has a metacritic score of 84 vs 94 for Halo 3.
84 puts it well below Batman, even with Battlefield 1943, behind Dirt 2, behind Shadow Complex (89), behind Guitar Hero 5, slightly behind Red Faction... here, just look at the list of games that have a higher Metacritic rating:
http://www.metacritic.com/games/xbox360/scores/
I think it deserves the 84 based on the 4 levels I've played so far. 84 is a score for a good game that doesn't have significant flaws, but isn't a masterpiece like Bioshock or CoD4.
If you asked if it would have sold 2 million copies if it had a different name or a different developer, my answer is hell no.