Notice how I said "in my
opinion".
The biggest thing for me is it seems pointless. The movie is barely even about the Basterds, and even Landa isn't really given much to actually do as far as an actual plot until the end. Waltz completely makes that character memorable.
90% of it between the first scene and the climax seems like a bunch of characters sitting around having witty conversations in various languages just to be showing Tarantino can write witty conversations.
Yea some scenes are good. The opening scene gets so tense it almost makes you physically uncomfortable. And the scene in the bar gets that way. But most of it doesn't feel like it's in the service of an actual story. The closest thing to a consistent plot is Shoshanna avenging her family, not anything to do with the actual Basterds, who are actually almost completely pointless, as they really affect little or nothing about the way things turn out.
And it's not that I just don't like Tarantino...I really enjoy the Kill Bill movies, for example.
IB just was a lot of "meh" to me. It just didn't do much for me besides Landa and the spectacular Nazi pwnage in the end, which was pretty epic.