I think the decline of The Simpsons was a gradual process, although to me, the transition into a general lack of quality becomes really apparent about Season 10 or thereabouts. The main reason for this was the show becoming increasingly 'wacky', but not in a humourous way. (The wackiness aspect can be seen earlier, but as the quality remained, it's not much of an issue, except perhaps as a portent...)
The thing with recent episodes, (As mentioned by others) is that semblances of plot seem to have disappeared in favour of a random cavalcade of allegedly 'hillarious' happenings that at the end, doesn't really make much sense,(But these episodes aren't designed for us to think about, sadly.) which greatly harms their replayability.. And worse yet, the clever humour of old has largely disappeared, so the 'balance' has been lost. I know it's 'just a tv show' and not a civilisation and whatnot, but it still annoys me. (Especially when people go on about how much better the 'new' episodes are. Please.)
