I feel like they somehow maintain some slightly "depressing" songs, but those are more polished today. The majority of the songs of Mylo Xyloto are very happy and positive. Parachutes was amazing, but the songs were very, very
raw and acoustic. X&Y was just the most epic album, beautiful melodies, lyrics that were fantastic, everything was perfect. Mylo Xyloto is like the turn of a page for Coldplay, or so it feels. It's so positive, so bright and beautiful.
I think people (the ones who are being extremely negative with MX) will change their opinions a little bit after it's all "sunk in". Music needs time to marinate sometimes
I think Mylo Xyloto is close to a masterpiece, but a very different one from their Parachutes/X&Y era.