Watched the latest one (latest on E4) last night, wasn't that impressed. I mean some of it was generally humourous like Sheldon's reaction to being spurned for the trip to Cern, Switzerland, but generally it was a bit tame. Again the fact that two of the four geeks are in relationships rather kills off some of the humour. Like in Friends where the humour is they are all hopeless in love, having some of them in long-ish relationships rather counters one of the main things that is funny about the characters. You can't see Raj or Sheldon entering or sustaining a relationship, Raj because he can't talk to women when sober and Sheldon because he has no interest. So they retain their funny side because what was initially funny about them still is, but now Wallowitz and Leonard aren't screwing up their relationships - well in Wallowitz's case he has got a woman who doesn't find him creepy and repulsive.
I think this will fizzle out, certainly my interest is waning. I used to love the series Monk, switched on to watch yesterday and it was a documentary delivery 'style' which I switched off not long in. Can't stand Natalie Teager anyway, bring back Sharona, but some of the bite it had has now gone. In the past Randy Disher would say something stupid or embarrassing, and like a father figure, Stottlemeyer would put him straight. Now it seems like the stupidity is forced and unnatural, as if Disher is trying to be stupid and not just a comment that wasn't thought out - one is funny, the other is cringeworthy.
That said I found Red Dwarf also lost its way after 2-3 series, whenever it was they stopped being on Red Dwarf itself (stolen) and ended up on that bug thing. They also ditched the original music which annoyed me and changed Kochanski.
Was it the writers of Father Ted who said they stopped after three series because they would inevitably end up covering the same material and jokes? Once you start changing the characters, actors, situations etc then I personally feel the comedy loses what made it funny/work in the first place. Scrubs is another that started losing it, Friends did but only after 5-6 series and still managed to produce some classy episodes where the change in situations were less evident as the main plot of the episode just had them all doing something that didn't identify them with partners/kids etc
I'll refrain from saying RIP BBT, but it can't be far off at this rate.