i don't know about you, but i like actors who are able to play diverse roles, like Gary Oldman. i always get annoyed when i see actors playing themselves in every movie, and eventually it just gets tiring and i lose interest.

examples:

Will Ferrell
Michael Cera
Seth Rogan

for will ferrell's credit, he has done some different roles- Stranger than Fiction, Winter Passing...

I agree with you for the most part. although, as long as the plot keeps me interested I dont mind too much, kind of makes me feel like their friends in some creepy way...

You can also see Will Ferrell's popularity declining if his box office performance is any indication, like Land of the Lost.

There are some actors that I think play similar versions of themselves very well though, like Morgan Freeman.

add Hugh Grant

He seems to always play the timid soft-spoken nice guy.