Something you should take into consideration regarding music (in general really) is that the internet now plays a much bigger role, and has essentially superseded radio when it comes to promoting, thus things like myspace and what not have changed the way music reaches it listeners completely, and thus what is played on the radio is a very small portion of what is reflected in the overall trend.
Heavy metal as a whole is probably more popular than it has ever been, even more so than the halcyon days of the 80s - you only have to look at the big festivals around the world, particularly in North America and Europe to see this, as well as more 'extreme' bands are actually charting - 'extreme' in a sense that they have shouted/growled vocals which, lets face it, in days gone by had no chance of charting, and as an aside, I remember when Obituary's The End Complete was the highest selling DM record.
In saying all that, the more things change, the more they stay the same. You could easily compare it to the 80s, where there were the monster bands on top of the pile - your Iron Maidens and Judas Priests - just as there are now, just that the styles have evolved (I'm sure some would say for the worse). Under that are there are the more moderately successful bands, and you can keep working your way down the chain.
Ironically, people complain about metalcore and it's offshoots now, but anyone who was around in 2000 will remember that at that time it was a breath of fresh air and 'the return of true metal' and all of that kind of hyperbole at a time when nu-metal ruled the roost. No different now, except now metalcore is at the top of the pile - along with your hardcore, which is essentially metal with hardcore vocals, lyrics and arrangements - and everyone is standing around for the next bandwagon to jump (and the same thing happened with thrash in the late 80s/early 90s, NWOBHM in the early-80s, you can keep going back and see the pattern emerging here.
My summation of the biggest problem of the past 5 years is that we keep on the endless recycling (and that is right across the board in popular culture) that eventually wears itself into dust before anything of much originality develops from it. I see this as being down to the fact that we live in a globalized world, with so much information to sift through (just look at the internet) that you are bombarded with.
Of course at the end of the day there are always going to be great bands/artists/music around, just a matter of finding something that relates to your particular taste.