What do Monster Squad, Loverboy, Sylvester Stallone, Back to the Future, Bruce Campbell, 80s Hair Metal, and The Crying Game all have in common? They all have been a punchline, set-up, or reference in Regular Show.
To get to brass tacks, this show is hilarious. A blend of slacker- humour and surreal plot lines, Regular Show is recommended viewing for all mature cartoon audiences. It tackles moral dilemmas without being preachy, it uses "random" humour in a way which drives the plot to non- random outcomes, and its light enough on its mature themes that it can nail a safe PG rating (maybe PG 13 if you're one of those conservative types).
However; where Regular Show really shines is in its total awareness of what makes adults who grew up in the 80s and 90s tear up and laugh out loud all at once: Nostalgia. One major example of this is an entire episode which is a parody of (and indeed, named) Over the Top. Another example is the constant montages set to old tunes from decades past, such as Hang Tough and Workin' for the Weekend. Even better are the tiny references which most people under 25 wouldn't get, and hold screen time for mere moments before you're left wondering "Hey, didn't I see that scene in Logan's Run?".
As I said, this cartoon is not for kids, but not because its racy and sometimes inappropriate. It isn't for kids because the funniest jokes are made for guys like me: 25 - 40 year old geeks who can pick out the references, and appreciate the parodies. For this reason alone I love this show, but even then it can still stand up on its own as an entertaining cartoon.
EDIT: Since the time of this writing, I have witnessed a They Live reference, the Death Star with sunglasses, and a scene where the characters go on an "acid trip" (they eat a concoction of spicy foods and then start tripping out) set to the tune of Mississippi Queen. This show just got even better.