2 reasons why everyone is so disappointed:
1) It was an hour and 15 minutes long, too long for any sitcom. You can't keep a show funny for that long; maybe a movie but not a normal comedy.
2) There was a ton of hype; no show could actually live up to that much hype, everyone expected this to be the greatest show ever and it's impossible for them to please everyone.
I agree it was disappointing, my other article stresses that point more firmly, but at least some of the blame has to lie on these two elements (along with the cliche'd insistence on bringing back all the old characters, which no doubt came from NBC management or some easily amused group of fans).