It's a case of writers trying too hard to be clever. They want to write the show in the form of one twist or earth-shattering event after another. The root of the problem with Grey's recently is that the writers are writing for events and not characters.
So when Derek and Meredith finally get together and appear to be happy, the writers don't know what to do after that because simply letting them be happy for a while isn't an 'event'. In order to let them be happy, you have to write for the characters because whereas cheating, torn-between-two-lovers angst, divorce, and reconcile/finally getting together are events, happiness is a state of being that lies totally within the characters and is not part of the external world in which the aforemetioned 'events' take place.
With George and Callie, it's the same thing. It started as an event...George moving past his thing for Meredith. That was followed by the event of the two of them engaging in their sham of a marriage(sorry, I hate them as a couple). That in turn was followed by the event of Callie deciding she wanted a baby. That in turn was followed by the event of Izzy developing feelings for George. Event, event, event. But no character development. We didn't really get to see George truly face the fact that jumped into a marriage with a woman he doesn't really love for all the wrong reasons, because that is ALL about character and doesn't involve any big events to promo in commercials during Desperate Housewives. And we didn't really get to see Izzy slowly develop feelings for George over time, it just sort of happened over three or four episodes. It's almost as if it's an excuse to end the George-Callie marriage.
And the Cristina/Burke thing? Their whole relationship started based on an accidental pregnanciy. Cliche television 'event'. I never really brought them as a couple because we never got to see them fall in love. They just decided to be a couple over the span of, like, two episodes. And we never really got to see Cristina face the fact that she didn't really love Burke. That's not an event, and it's a character-oriented crises - 'holy shit, I don't love the man I'm engaged to'. That's good potential for a story actually, but they didn't go in that direction. They wanted an event. So what we got was Burke acting, uncharacteristically, like an idiot by not seeing that Cristina's body language was screaming 'I don't want to marry you' in the days leading up to the wedding, and then we got the wedding itself, which was a totally stupid excercise in avoiding the issue of two people calling off a wedding because they don't love each other by having Burke just decide that he doesn't want Cristina to have to 'try' to want to marry him, out of the blue. And at the end, Cristina breaks down and cries, like she just realized how much she loved Burke. The exact opposite of what should happened - that is, that they should have dealt with the fact that they never really loved each other enough to marry.
It's character assasination. It's bending characters to fit the plot, and that is the exact opposite of what the writers should be doing, and is, imo, the core of the problem.