I think Coldplay has two main flaws:
1) The lyrics. They're... ok... but... a lot of times they are just not good. For example, on AROBTTH, God Put A Smile... simply has bad lyrics. They seem... embarrasing. Even in their best songs, they never quite hit the mark of "melting everything"... emotionally, I mean. They're... just... ok.
2) The music... is almost always beautiful, but boring. They don't do much dynamically, and they don't stray from their verses and choruses and bridges, at all... and their songs seem too long and too clean with no variation for this sort of ruckus. Just my opinions.
I do love A Warning Sign, though. Man, that song is brilliant.