The fact that most of them are bad isn't really a beef so much as an observation.
Good (I'd watch again):
Xmen 2
Spiderman 2
Mediocre (Not bad, but I wouldn't watch again):
Spiderman 1
Xmen 1
Batman Begins
Fantastic 4 Silver Surfer
Bad:
Batman pre-begins (like 3 movies)
Hulk
Fantastic 4
Catwoman
Daredevil
Xmen 3
Spidey 3
Electra
Punisher
A ton I'm forgetting
Not very balanced at all.
It's really simple why they aren't good, the genre of 'Superhero movie' is in the same category as 'Sequel,' in that the movies are helmed by people whose only interest is to make money, the only reason the films are ever made is for money, and attempts to make a good film aren't emphasized because everyone is already going to go and see the film. Spiderman 4 could be the worst movie of all time, but if they release it in every theater in America it'll make 200mil opening weekend. Obviously movies are made for money, but with a lower-budget flick you might have directors/writers/actors that really care about the project and make it well, especially with people who both write and direct (Tarantino, Coen Brothers, Kevin Smith, Spike Lee, Scorsese, etc).
It's called a pre-sold franchise. It's why 99% (The only exception I can think of is Toy Story 2) of sequels are worse than the original but make more money, the movie is sold before anyone even goes to the theater.
When you have a superhero movie, there's not much reason to make it 'good.' No one goes to a superhero movie to see 'good,' all anyone wants to see is X hero from X comic book on-screen doing cool CGI and dishing out witty banter. The problem is that most superhero movies spend much more time 'developing' characters than fighting, and it gets boring. Spiderman 3 is a perfect example, the movie is 3 hours long with 10 minutes of CGI, all I went into that movie wanting to see was Venom and Spiderman fighting, and by the time that was onscreen I had literally left the theater, I came back later to see the ending before some other movie started. Some just fail for so many reasons you can't really count them at once, like Xmen3 and Catwoman. But the bottom line is they tend to suck. If there's anything the trailer for Iron Man is doing right, it's showing a ton of action, but then again the trailer to Spiderman 3 wouldn't have us believe we were walking into dawsons fucking creek, which is what the first 2.5 hours of that movie turned into.
/rant, most of them are bad. I have a 'beef' with bad films, not superhero movies. I honestly fear some of my favorite superheroes getting their own films though, god knows how much they'd fuck up the green lantern.