Hi mate, yes out of tune, as in slightly off pitch instruments drive me crazy, but if someone is playing a song in a different key then that is fine, no problems with that. I often tune everyones instruments up as i don't use an electronic tuner and find it quicker to tune by ear, i usually play a "G" or "D" open chord and tune the guitar to that because i find with a tuner, its accuracy depends on the Height of the guitars "action".
I used to have a mate that would ring me up and i would tune his guitars over the phone.
i use different tunings a lot and have three of my guitars tuned differently at all times, one is tuned at 440, one a tone lower and one in DADGCD, which is completely different from standard tuning, so no problems with different states of tune. As far as communicating riffs to others, i would generally play what i want them to play before we jam and take it from there. I can "read" my guitar very well, I know every note on the neck, so i can still relay chord changer verbally if i have to. My timing comes from being able to play drums, i find this invaluable for timing and "FEEL" for a rhythm as it allows me to play around the tune but still stay in time. I would be totally lost without my music as i play every day, it is a great release for me.
Hope i covered your questions.
Mick