Recognize this guy? That's Jack Black. You're probably familiar with his body (to forgive the pun) of work. And if you've seen a few of his movies, you probably have a strong opinion about him, one way or another. And if the internet and box office sales are any indication - there's a good chance that by now it's a negative one. It's certainly not a guarantee (I personally am still fine with him, though I prefer him in small doses), but I've noticed that a certain subset of comedic actors, specifically ones that play a 'man-child' character regularly - men who are either inept, immature, lazy, or moronic, or some combination of the four - ultimately find themselves out of luck as leading men in a relatively short period of time.
Actors like Denzel Washington, Harrison Ford, Jack Nicholson and Robert DeNiro can pretty much play the types of characters they're known for (the strong, silent type, or the strong, loud type, take your pick) forever. Audiences expect it, they get it, they're happy. But comedic actors who play very heightened characters - Will Ferrell, Mike Meyers, Adam Sandler, almost anyone who's come from SNL really - start at a distinct disadvantage. They can enjoy success with a handful of movies, but once audiences learn their schtick, they seemingly have three choices: go dramatic (/tone it down), go animated, or go home.
You could argue that this was Carrey's choice - he had done comedy for over 10 years, and wanted to show off his acting chops in more serious, dramatic roles. And it's certainly possible his desires lined up with what needed to happen for him, career-wise. But either way, I'd argue that if he continued to play only comedic roles similar to where he started, his name would be nowhere near as well known as it is now. Let's return once again to Jack Black, who actually has a pretty fantastic resume, playing a supporting role in a LOT of movies and television shows over the course of 20+ years. But his leading man status shows that since his first big lead role as Shallow Hal in 2001, he's been in more flops (Year One, Tenacious D, King Kong, Be Kind Rewind) than successes (Tropic Thunder, Nacho Libre, which IMDB hails as a success, to my surprise). Black is the best example to my knowledge of a known comedic actor who for the most part has stuck to his guns long term, for better or worse. His only notable recent success as a leading man comes in the form of an animated feature, Kung Fu Panda, (and likely, it's forthcoming sequels).
Looking at other comedic actors who play less man-child, and more "impish adult", you have your Ben Stillers, your Owen/Luke Wilsons, your Vince Vaughns. These actors have either mixed up their resume from the beginning, playing a combination of dramatic and comedic roles, or they've generally played characters who, at worst, just need a shave, a kick in the ass, and/or an attractive female lead to get their act together, thus avoided the potential comedy pitfall from the beginning (as a side note - as far as I can tell, Vince Vaughn is generally playing either a more irritable or a hornier version of himself) To his credit though, Ben Stiller has both pretended to have his penis get caught in the teeth of his pants zipper and allowed fake semen to appear hanging from his ear, so certainly some credit goes to him.
It's possible that it's just an age thing. Audiences may be willing to accept the man-child performance from an actor in his early to mid 20's, and less if they're older (though I think most of the actors I've listed were doing it in their early 30's, in fact), where from a believability standpoint, its just harder to buy these men surviving in a world so mentally and emotionally stunted. I recently saw David Spade in a scene in Rules of Engagement deflecting all common sense about a woman blowing him off, and seeing him play a middle aged man so completely in the dark struck me as less comedic and more sad than anything. Having said this, Steve Carrell, who I personally love to death, got big in his 40's playing a 40 Year Old Virgin, very recently played another man-child in Dinner For Schmucks, and continues to play one of the most emotionally immature men on television in The Office every week, so he may very well be the exception to the rule.
These actors are of course all adept at their craft, and with the exceptions I've listed I personally still enjoy most of their work - but I find it fascinating to see how audiences slowly turn on them. Often the ridiculous characters these actors play in movies start off as supporting characters - a la Jack Black in High Fidelity or Saving Silverman, so when these characters/actors come to the forefront, it's interesting to see if and how they tone it down.
It's still a bit too early to tell where Michael Cera, Jonah Hill, and Seth Rogen (soon to appear in his first semi-dramatic role in The Green Hornet), and many other of the younger, budding post-Apatow comedic actors will wind up eventually, but I'm certainly interested to see if this trend continues.
PS: My mother's always a good litmus test for where the actor stands with the general public - if she likes him: "he's so silly!", if she doesn't: "I like that other guy more," or...silence.
PPS: Wow, weird timing of this post - I wouldn't have ever thought to mention Leslie Nielson, and I'm not sure where to fit him in my thesis, but for now I'll just say he definitely used to make me smile, and for that I'm grateful.