I do not believe metal is the most 'complex' genre or the genre that can send out the 'most emotions'. Anyone telling that to either themselves or to others is just completely lying. Yeah, I see that metal is generally 'faster' than a lot of other genres, but in terms of 'complexity' for example, it doesn't hold up to classical music of the 18th century or to bebop of the fifties. In terms of technicality, there's also other genres that are far more 'difficult' to play, just ask any classically trained musician, for example.
That said, I do not mean to bash metal as a whole, since I do like quite some metal, but I have never considered myself to be a 'metalhead', not even back when I had my hair halfway down my back. Because, here's the thing: the majority of metal is fucking BORING. Seriously, it's all the same! It's not more difficult to write a metal song than to write a catchy popsong. In fact, I'd say it's a whole lot easier. You just take a riff and add another and you're done. Now, that's probably oversimplifying it a lot, because metal, like every other genre, has it's qualities. It's aggressive, which is something that can either be good or a turn-off for the listener and it's usually fast; same thing applies.
But now here we go; there's also this insane amount of subgenres within metal, that makes it very difficult to pinpoint exactly where or what makes it good or not good. I could say here that metal is 'melodic', but then you could turn on your average death metal song and not find the slightest hint of any melody whatsoever. I could say metal is fast, but then you can turn on a song by any random doom metal band and my statement would be proven false.
So, metal isn't necessarily complex, nor is it technically more difficult to play than any random other genre, nor isn't it more difficult to write songs etc. etc. Again, saying it is is only an elitist approach and in my opinion shows a very limited notion of what else there is in terms of music. That said though, 'metal' as a whole can probably be regarded as the 'heaviest' type of music available, though there would be also fanatics of electronic music who would disagree (I'm looking at drum 'n bass, hardstyle etc. for example). I think the main thing that's going wrong with questions like this is that metal is too varied to give ONE clear-cut answer. I do like metal, honestly, but there's also a shitload of the stuff I don't ever want to listen to.
I think the main point of attraction in metal is the huge variety of different subgenres, but then again; don't we have this same trait in any other style of music?