I would probably say that any distaste or thumb-nosery (yeah I just made that word up, deal with it) towards the metal genre may come from its lack in innovation, and (depending on subgenre) inherent lack of melody, or thought provocation. Now I know I'm going to upset some people here with the previous statement, but cool your jets for a minute. I grew up as a metalhead, and the most influential music to me was in the genre. I'm not some Johnny-come-lately to the party. I'm coming from a place of love
From speed metal, thrash metal, death metal, satanic metal, comedy metal, doom/sludge metal, industrial metal, Christian metal, rap metal (which should never have existed), goth metal, etc....... you get the point, I've listened to and loved bands from ALL metal genres, and still have a huge sweet spot for all those bands I used to listen to.
But then one day, I started liking softer music. Music that seemed to have a different feel to it. What I encountered was, music doesn't have to always be aggressive, and music isn't always about negative things. Melody can actually be really beautiful and emotive when it's not limited to dueling guitar solos, or to the "slow" chorus (there are exceptions to these rules, and I could also name bands that are, so I am aware).
Metal, even in all of its subgenres, is too often limited to certain factors, and can be too formulaic- it's just inherent in its genre. And even though there are a lot of talented metal players that are extremely technical, technical doesn't always equal good.
So where I stand, a lot of the genre including sub-genres has been done before, it's just a new set of faces. Other genres are not as limited to how they can create, because they don't have to follow by such narrow rules. It seems the majority of metal bands these days follow the narrow path of how to play more aggressively than the last band in that genre, or play more technically advanced than the last band to hit it big within that genre. The rest are left standing in a puddle of "it's been done before".
I'd be curious to hear people's opinions of bands that have advanced the genre in the last 8-10 years, or brought something new and innovative to the table, but I have my doubts.