I agree about Nickelback and their ilk, Slayer (definitely my least favorite of the "big 4" of thrash), and definitely about AC/DC.
I think Boston fits this too, but not quite to the same extent. They have a lot of recognizable elements in basically all of their songs (vocally, the guitar harmonies or whatever they are, etc), but that's common to a lot of bands, and I wouldn't say it necessarily means they're all too similar.
I'm a fan of Tool, and I wouldn't say that many of their songs sound similar, but rhythm-wise, there's a "tribal" drum thing going on in a ton of their songs (and it seems like a lot of the same "complex" rhythms are used) and is getting pretty old (sorry, again, musical retard here).
Disagree about Iron Maiden and "all music with growls". Even though I think there is variety in harsh vocals, they aren't as variable as normal melodic vocals, simply because that entire element (melody) has been greatly diminished or eliminated altogether.
-J