There's some really good writing early on it seems.
The episode I saw last night had references to the change in theme song and even went back to the one used for season one to close the show.
I prefer the original theme song, it gives the show a more serious, more somber tone which I think is appropriate for the subject matter.
I like the original theme, but I wouldn't have said it sounded particularly serious, and definitely not somber.
Then you must not remember it, the original theme is a melancholy jazz piano composition which the show would play whenever Monk was depressed for some reason or another. And the original opening credits with Monk getting ready for his day to go see Dr. Kroger gave the show a downbeat feel from the first moment, especially in the pilot episode where you can hear Monk talking to himself while he prepares for the day. Music ALWAYS dominates video, if you have the happiest video ever filmed, but put a really sad song over it, the video becomes depressing. And those original credits are depressing.
Honestly, the only TV theme song I can think of that is more depressing is the sad little song that used to play over the end credits of 'The Incredible Hulk' where you see David Banner hitchhiking. Man, as a kid I practically wept every week seeing that....and the name of the song? Appropriately enough....'The Lonely Man'.