Shippery
One epic thing about Doctor/River is that she seems to know what he's actually like, rather than looking at him as a Generic Male Of Her Own Species. She seems to think of him as both The Oncoming Storm and as her personal bitch, which is a bit complex. I am not sure which of them would blink first in a staring contest. He is all trying to reassert some control by flirting back, because she throws him off-kilter and confuses him. He is scared of what she signifies and at the same time he wants her "liek teh woah."
Nothing says "I love you" like blowing up her planet or crying over his dead evil body, but apart from those things it is quite epic to watch someone die for you and then decide that you want to shag them anyway. I like seeing it develop and not just being instalove which is often dull. And you get all sorts of nice timey-wimey wonderings, like is he predestined somehow to love her because she died in his place and let him carry on living long enough to meet her again? Like you know how the Doctor is now sort of literally "Amy's Doctor" after she remembered him back? Is it a bit like that?
And he always has to turn up when she calls him, in case she dies before he can ever meet her in the first place. He is a bit torn between "I am not her bitch" and "wheeeeee my girlfriend wants to see me!" They are passing notes to each other across time'n'space and I expect sometimes the notes are quite explicit because River is not shy.
A thing about not knowing who the fuck River is, is that she must be a bit scary herself not to run screaming from the Doctor as she knows him. Maybe she is too distracted by the oncoming storm in her pants, idk.