Paul Rudd's rise to leading man status has been the most gradual rise to fame since Andy Warhol's "fifteen minutes of fame" line. It would make more sense to compare Rudd's career to that of a professor or author than other Hollywood actors. For most that have Paul Rudd's name recognition, the pattern goes like this: somewhat prominent TV role, then a sharp rise into the movies, either jumping straight to leading actor or doing a movie or two as the second fiddle before taking over. After that, a long mellowing period of getting roles on name recognition, even though the actor has more or less entirely checked out. This describes the careers of Ashton Kutcher, Jennifer Aniston, Jack Black and, well, a lot of people. Some age more gracefully, some have a different start, but I'm concerned mostly with the shape of the graph: sharp rise, followed by a lot of movies to remind the public that you still exist.
Paul Rudd, on the other hand, has been quietly and persistently reminding the world that he exists, and, like a coworker whose name you finally get after hearing it 18 times, he has finally stuck. He managed to get famous without a single moment when all of a sudden everyone who was going to know who he is did know who he is.
Consider this: as of this year, Paul Rudd's film career is old enough to drink. And when, exactly did he start registering on your radar? Sure, you might know he was in Anchorman, but is that when you started recognizing him as something more than "that guy?" How about 40 Year Old Virgin? My first real "Paul Rudd exists" blip came on Knocked Up. I'd seen him many times before, but I didn't realize it. For whatever reason, the guy, for all his talents, is immediately forgettable.
Maybe you knew he was in Clueless (I didn't), but how about Romeo & Juliet? The Cider House Rules?
This Is 40 is probably Rudd's most prominent role: he is the leading guy in a popular comedy, and the movie is actually a lot like Rudd's career: a series of thematically-related scenes based around a particular character. It's funny, well-written and generally forgettable.
Now go look at Paul Rudd's imdb page and appreciate the glacial rise of The Master of Being Around.