Reading online about this season of Regular Show, the consensus appears to be that the show has gotten stale and that all the episodes are essentially the same in terms of their structure. While I agree with much of this, I wonder how it took five years for people to come to this conclusion since essentially it has been this way from the start; basically Mordecai and Rigby get asked to do something, they mess it up to a major degree, which in turn sparks a race to fix it and/or avert a bigger disaster – which usually involves the appearance of a random and often supernatural character or aspect. This is the basic formula of the show, and to suggest that no other shows have formulae they stick to is daft. The difference with such shows working or not, is that they need to deliver on this formulae while also having some form of forward motion, and keep things fresh and engaging enough so they do not feel stale.
The fifth season of this show remains as solidly on formula as it has been before, but I do agree that this season has a certain feeling of being stale about it. It probably didn't help that I watched this show after watching the most recent season of Adventure Time. Sure they are different shows, but while Regular Show seems content to essentially change the details but keep repeating some base formulae, Adventure Time is expansive, interesting, innovative and genuinely engaging. The lack of movement within the show is a challenge, and I did feel it this season for really the first time. There are still plenty of laughs, and the creativity of some of the specifics is fun, but too much of it feels interchangeable across episodes, and it does tend to merge together (particularly the episodes which don't stand out – which is most of them).
I will still return for the sixth season, because I do like the formula that the show is working to for each episode, however if it is a continuation of this fifth season then I guess that would pretty much be as far as I took it.