He's extremely full of himself. Have you noticed he pretty much just shows up to the Grammys to collect his yearly prize for his freshly released, adequate albums?
He's making a ton of money between touring, releasing albums, and nowadays it seems like a camera is following him around everywhere. Not that he's greedy necessarily, but he certainly has found a sweet spot with his fans and the music media.
He's the ex drummer of nirvanna, and I think that has a TON to do with it. I personally don't like the foo fighters but their first few albums aren't bad; it just seems like the rest of their work sort of slumped into this very okay state where it can be pushed out every year or two for some easy touring. I think these guys enjoy being a band and having fans more than making music, but I also think grohl is pretty full of himself.
This is also the same band that once joined the anti Vax train and told people not to vaccinate their kids at benefit concerts. Implying that they are smart enough to know what they are actually doing is a little much. I believe they just found a certain method (that's a little trite if you ask me) to pump out these alright records which happen to be better than entire top 40 pop scene (as if that was difficult) and so they are well received by "critics" and the oh so "impartial" Grammy judges.
I think this band cares more about being relevant and about its reception than its artistic integrity and meaning. There's not a soul to their music because it's just a vehicle for them to feel good about themselves and have fun in their 40s. People that look up to Dave grohl as some kind of musical hero have entirely missed the point of music and art entirely, in my opinion.
However, I will concede that he was great in nirvana and the early foo days, and as it stands he is far better than any stupid pop "artist" because he actually writes his fucking music and plays it too.
He also has the balls to sing straight to tape with his latest records, vs. all the nasty post processing and overzealous digital mastering in use these days. Anyone that fights the use of auto tune and loudness wars 2.0 is alright in my book. I believe his last few albums have all been fully analog recorded in his garage, but feel free to correct me on that.