Okay, Die Hard 4 was good, but to me it had lost some of its innate Die-Hard-ness. No vest, Willis is totally bald (okay, had to happen sooner or later), he has a sidekick (not Samuel L.), he doesn't talk to himself enough ("Come out to the coast, we'll get together, have a few laughs."), the motivation for the bad guys is shit...it's fallen to 'just another action film' level for me.
Not that it doesn't have some great action scenes, it does. That might be part of the problem, as the series has got more spectacular with each film.
Missing the secret DH ingredient, whatever it is.