Indeed, cogito. That's a great post. Nothing much to add really.
To tackle it from a slightly different angle: there is the physical and the mental part that intertwine so closely and which eventually bring about the joy.
Being intimately close and still living a scene which includes dominant and docile aspects separating the lovers from one another can work miracles.
For the more graphically orientated mind: really there is nothing better than a woman's round naked butt being shown to you - and being available to you for your (and her!) enjoyment. A warm surrounding - be it a summer day in the backyard, a secluded part of a little bosk or a well-heated bathroom - and you can take it real slow. I enjoy making her undress and kneel with her bottom up in the air, parting her legs slightly and spreading her cheeks for me. Then leaving her for a while while she has to hold her ass open. Part of the excitement is that (at least theoretically :-) a stranger may walk by and see her giving such a lewd display (of course, I make sure that nothing like it will happen and of course, she knows).
Just a couple of minutes usually do and I have to hold back watching her while I am a few meters out of her sight. She doesn't obey my order to keep her ass nicely parted but starts to masturbate. Again, the real fun takes place in our minds: she knows that I watch her and I again know that she knows but still puts on a show for me. So part excitement comes from the reality of the kinky situation and part of the fun is just fantasy. I think this can only really work if you trust one another.
Naturally, we like to continue the scene. She doesn't like to be lubed too well but enjoys the raw feeling of just a saliva or pre-cum coated nozzle entering her.
(Apart from that everything has to be pretty comfortable and just allow for the illusion of real dominance and submission).
I enjoy getting a blow-job (hey, what a surprise) when I make her hold the enema. If she likes me to take it easy on her, a mid-size butt-plug helps her retain the enema. But I also like to make her hold it without such assistance. Her squirming not to leak is good fun and making her hold her ass cheeks apart to "train" her ass properly adds some enjoyment.
If possible we like to cum simultaneously while she expels. The combination of heavenly sexual arousal and the illusion of humiliation or complete control respectively make for some wonderfully intimate time to remember.
I completely agree with another post that I saw elsewhere on the site: the brain is the hottest erogenous zone we have.
Well now, this has become rather lengthy. I hope you still appreciate my thoughts.