My main problem with the Jane and Casey relationship is that is is very, very poorly developed with lackluster chemistry between the two actors as well as the two characters. 5 years of Casey and I still don't really know his character at all. The reason behind this is:
1. We never see how Casey and Jane met, which I believe if the writers were truly committed to this relationship, they would show. Because of that, there is no kinship formed with the couple because the audience never really knows what their dynamic is.
2. Casey is only used as a plot device. Whenever the story lags or there isnt any movement, they bring him in to stir the pot. He asks Jane to come with him, or asks her to be with him, she is usually unhappy because of some or the other contrived "complication", then he leaves, she's sad for an episode and then she goes back to hanging out with her family and best friend. Wash, rinse, repeat.
3. Now, this is my opinion entirely, but Chris Vance and Angie Harmon have awkward, lukewarm chemistry. Neither does he seem on the same wavelength as her as an artist, nor as the character. When Casey comes, Jane Rizzoli a fairly unique character, comes across as either a totally different, cookie-cutter woman with no gumption or then she seems kind of disinterested and aloof. I can't tell if this is the writing or the actors, my guess is a little of both. And Jane acts the same way with all romantic interests (the way Maura acts exactly the same, yet out of character way with her serial killers/love interests).
4. The way Jane and Maura are written - I'm sorry, but best friends, no matter how close do not act that way with each other. (See: Meredith & Cristina on Grey's Anatomy, Rachel & Monica on Friends, JD & Turk on Scrubs etc.) The gals on Grey's always lie around in bed together, eat every meal together, stay in touch all day, prioritize each other over their love interests - and yet MerDer and Owen/Cristina have been two of the biggest couples on primetime TV; nobody ever doubts the Meredith/Cristina relationship is anything but friendship. The way Jane acts with Maura is so, so vastly different from the way she acts with Casey and I can't tell if this is intentional or simply a product of the actresses' chemistry with each other. Lingering looks, constant touching, suggestive smiles, possessiveness and protectiveness are a few traits which the Casey and Jane relationship should have but other than sex they don't have any of these. Instead, the writers have made Jane and Maura have constant interactions of this variety. Right off the bat, from the pilot, the average audience compares this show with Bones and Castle, its leaden with 'Will they, won't they' potential. And I think this hinders the ladies' relationships with anybody, even a man as sexy and attractive as Casey.
Now, I am not a Jane and Maura shipper, but to be honest - it is difficult to see their interaction or their chemistry as anything but suggestive of something beyond mere friendship. I initially did hope Casey stuck around, because, lets face it - the guy is hot :P But now I'm thinking he seems less hot when he's with Jane.
