I think that the Jim-Pam relationship has changed considerably since the end of Season 2 and it's not because they're together, but i think that that's taken down the show a little.
At first, Jim and Pam were these two people who in the midst of everyone were just right for each other.
Now, it's like Jim and Pam are the two most popular and in-demand young people in the Office. Jim isn't this guy who's akward around women and is interested in that one girl because he makes her feel complete. Since his successes with Katie and Karen, Jim's like this guy who can get anyone he wants.
He's no longer the everyman I root for. If Jim got rejected a little more, or proved to have some social kryptonite. I just see Jim as the alpha-male, now. For one, he used to pick on Dwight because Dwight annoyed him but his treatment of Andy is uaully unprovoked. But Jim had an, "I gotta show Andy who's boss," about him when he started messing with Andy by stealing his cell phone. Jim does what he does with Dwight because he works in close proximity to him.
And Pam is like the alpha-female of the office. Now, it's been revealed Ryan seems to have had a crush. Creed has seemed to have a sexual fetish for her. Toby likes her. She's like the most desirable property in the office.
Also Jim is like a big guy in size and he has cool hair. Those guys are usually the ones I see getting girls in bars. Girls generally are physically attracted to bigger guys. Martin Freeman, on the other hand, was small in stature.
Jim used to be the hopeless romantic, but I think he's like the alpha male steaking out the biggest property in the Office. And when you see it that way, then Jim's misguided relationships with Karen and Katie to get to Pam seem a little less noble and kind of wrong.