1.) Harrison Ford is old. That could still work, however not only has he looked old in his last few movies he's also looked befuddled and lost.
2.) Shia Labouf. The guy's a terrible actor. He plays the same character (see the awful Transformers for his template) in every flick. He believes cocky and arrogant channels Brando or something. How cliche is it that he's playing a biker dude? This is supposed to be Indy and Marion's possible love child?
3.) Aliens? I hope the reports aren't true.
4.) We often forget that the only true classic Indy movie was "Raiders of the Lost Ark." The Temple of Doom had some iconic moments...but overall the movie's pretty dumb. Kate Capshaw...one of the worst movie heroines ever. The Last Crusade was a little better...but still sappy compared to Raiders. It's hard for lightening to strike twice.
5.) George Lucas's writing has gotten progressively worse as the years have worn on. His dialogue for the Star Wars prequels was awful.
Again, I truly hope I'm proven (or proved!) wrong and that we'll have another classic this summer. I'm hoping if I keep my expectations low I'll be pleasantly surprised.