It's quite normal for Japanese dramas, and anime to fall in quality from midway in the show, let met name a few:
Planetes - I enjoyed this so much the first 12-13 episodes,but then Hachimaki decided to become a fag, and the show got boring and too serious.
TTGL - It wasn't as a playful,nor as appealing after the time-skip.
Nodame Cantabile - Got too serious after the first concert performance by Chiaki
My Boss My Hero - Lost its sense of humor after the first few episodes.