You are bang on Magnet.
I'm a burgeoning Iron Maiden fan, but I hadn't heard "Wrathchild" before. I loved learning to play it and still find it a challenge.
That's the bonus to having songs by big name artists that not everyone will know. You'll get a good quality song (More often than not) and you'll also get a challenge that will interest a great many people and keep them playing the game.
jlc510, you're right. If the setlist was different for GH we might not be here today. However, now that it has established itself and has a pretty dedicated fanbase it should be taking that extra step and providing something more in terms of the musical choices. Choose the more complex and fun songs rather than the standard greatest hits from artists. GH III most certainly isn't taking this route and neither is RB at the moment (To be fair, RB does need to build it's fanbase from scratch as it doesn't have the name recognition yet. As well, all the songs haven't been announced yet and DLC will undoubtedly add a lot of songs that may not be incredibly familiar to everyone)