sgamadison suggested that perhaps I should explain why I like sub!Rodney so much. At the very least it can't hurt and it'll get my thoughts and feelings down, so:
As I've mentioned before, I'm not really into the more intense aspects of BDSM. I do like reading about spanking and bondage (and being held down/pinned in place), but I'm not into whipping and flogging. It's really the relationship/emotional dynamic I enjoy. I like reading fic where John can tell Rodney to go to bed, and Rodney does, because he's been working 16 hours straight and won't stop unless he's made to. I like the idea of John helping Rodney get out of his head for a little while - Rodney lives in his head so much.
It's all tied up with my head!canon for Rodney, I think.
In my head!canon, Rodney spends much of his life unwanted for himself. Canonically, his parents fought and blamed him, so I don't see much support or love coming from there, and we know he was bullied in school. WE get a glimpse of a pretty crappy childhood. (My OTP is Rodney/Happiness).
Toppy!John would take care of Rodney when Rodney probably hasn't received much of that kind of attention before. John would able to make sure Rodney gets some sleep, even when he's exhausted and belligerent. Rodney would listen to him, because John's his top, even if Rodney would really rather stay and work on whatever it is. John looks out for his sub, and he clearly feels Rodney needs sleep, so Rodney grumbles and complains and reluctantly gets some sleep and is better for it.
John can withstand Rodney's sarcasm and rants and anger and stand firm, to be what Rodney needs. Toppy!John would help Rodney relax and encourage Rodney let go; part of that "letting go" means doing what John tells him, trusting John to take care of him and have his best interests at heart. (The idea of Rodney just doing what John tells him is also very hot).
Letting John take control means Rodney doesn't have to worry. He can set aside the stress and constant thinking (always thinking, thoughts racing, bouncing between what can go wrong and what to do if it does, what needs to be finished, theories and numbers unspooling, considered and discarded) and just be in the moment.
"Clear blue skies, all my troubles just drifting away..."
When he steps out of the bedroom, he can shoulder the responsibilities with a sense of renewal. It's like running and running and running and finally getting a moment to take deep, calming breaths before you start the mad dash again. Remember the episode Submersion? John and the team defeat the Wraith Queen and then take a nap. Rodney? Rodney goes back to being CSO. I'm not saying Rodney was unhappy about it or that the team didn't deserve naps. But he's learning and utilizing military skills on top of running the science department (receiving reports, making sure nobody blows things up, allocating resources and assigning things) as well as completing his own experiments and research. The military aspects are all new to him.
Rodney a scientist that's new to the stress of being on the front lines. Relaxing is probably precious, rare, and hard won, because Rodney can't turn his brain off that easily.
It wouldn't be easy for John to get him to chill, but I truly believe it'd be very rewarding for Rodney once he's in the head space to let John take care of things. John would help Rodney settle into himself, make him feel comfortable in his body (not just comfortable with thinking).
John: Look, if you have to keep asking, you’re obviously not getting it. This is about letting go.
Rodney (almost in tears): I don’t know how to do that, OK? I’m me -- I don’t know how not to be me.
But he manages it in the end, so obviously this is something he can do. If John makes him focus on the physical first, to pull him out of his head, and then go from there - because Rodney McKay lives in his head, and the hardest part for him would be to intellectually let go. Various fics have posited part of why Rodney is so abysmal at hand to hand combat is he can't stop thinking.
So John gets him to stop thinking.
And the rest follows.
Basically? I want John to love Rodney like Rodney's never been loved before. I want John to have someone to come home to, who gets him, who slipped past the barriers he's thrown up, a relationship that shouldn't be so easy but is. John gets something good, something that makes him happy, Rodney gets something he needs that makes him happy and relaxed, too.
And the have absolutely filthy sex.
I find Rodney McKay hot like burning, and I'd like for John to feel that way too. I want John to fuck him, rim him and make him beg and come all over himself. I want John to make sure he eats and doesn't have a breakdown from stress.
Rodney McKay getting fucked is hot as hell.
(Does this even make sense?)