Too much exposition, and not enough Basterds.
This film was bogged down with too many plots to kills Nazis, but very little to no character development was given to the Basterds. Sure, Lt. Aldo Raine (Brad Pitt) had some clever lines, but it wasn't enough for me to care. Quentin loves his long winded conversations and witty repartee...we get it already. I do have to admit that the dialogue was well crafted, but it just wasn't adding to much.
People will say that Tarantino's always been about long verbal exchanges, and snappy one-liners, but in the past those words were creating interesting characters or adding to the plot. Here, they did help create some tense moments, but I just felt there wasn't enough payoff.
Oddly enough, the characters that were fleshed out the most, and were easier to sympathize with were the "bad guys". Christoph Waltz brilliantly portrayed Col. Hans Landa (aka the Jew Hunter), while Daniel Brühl played the lovestruck, German war hero Fredrick Zoller. These are the only two characters that we really get to understand.
I guess my biggest gripe with this movies was I wanted to know more about the Basterds; who they were as people, and why they were motivated to do what they did. By the end of the movie, I cared more about the Germans than I did about the Americans & the French combined. Maybe that was QT's goal, but somehow I doubt it.
Was Inglourious Basterds Tarantino's worst film to date? No (that title still belongs to Death Proof), but Basterds is far from his greatest work.