Summary of what's known so far

From what I've seen around and seen posted on Reddit, since there doesn't seem to be an obvious info page here

*First spin on a previously un-spun stop gives 250 xp
* Gyms have a badge associated with them, which has a bar at the bottom; this represents progress towards leveling up the gym badge. Level it up by interacting with the gym (feeding berries, spinning stop, defending, and attacking all seem to do this - battles actually show a number representing the amount of "gym badge xp" or w/e you call it gained.
* Gyms act as pokestops too, but start by only giving 2-3 items. You get an extra item if your team controls the gym. You also appear to get extra item(s) if your gym badge has been leveled up.
* Gym battles are currently very glitchy, with energy and health bars jumping around, likely due to server loading, and switching mons is slow and glitchy.
* Combat mechanics appear unchanged other than the type effectivness changes (which are a huge change). Gengar is significantly better on attack now due to his huge normal resistance, things with double weakness are worse on defense.
* Pokemon just placed into a gym don't start with full CP - you need to feed them a berry to get their full CP.
* Motivation is a multiplier applied to the base CP.
* Motivation drops at a rate of ~1.1% / hr for pokemon whose CP (at full motivation)is 2999 or less, ~10%/hr for pokemon whose full CP is 3000 or more (even if their current CP is lower due to lost motivation). There seems to be a lot of pushback from community on this, and people are hoping niantic does something to make this a bit more reasonable.
* Feeding berries restores motivation, but there is diminishing return as more berries are fed to one pokemon - the second berry restores much less motivation than the first. The effectiveness of berries returns over time. It's not clear whether this decrease in effectiveness for multiple berries depends on who feeds the pokemon, or is the same for all players who might feed a given pokemon.
* Feeding gives 20 stardust (at least with white/bronze badge). Scattered reports of people receiving candies for the pokemon that they fed. This appears to be rare.
* Defeat in battle takes ~20% of CP. It takes ~5 defeats to kick a pokemon out of a gym from full motivation; the later battles are much easier and you can often sweep the last two rounds with a single strong attacker.
* 1 coin per hour defended, rounded down. No dust.
* Raids are currently in, but lvl 35+ only. This will likely change as the kinks are worked out.
* TM's are available from raids, and they do work the way we expected. You can get things like DT/DC dragonite.
* There is a delay after a gym is claimed for a team before other people can slot in.
* xp awards for battling may have changed. They are displayed from the gym screen, and sometimes don't seem to be shown at all, making it difficult to figure out what's going on.
* Tapping a gym badge from player info screen works now (it was busted yesterday) and gives you some cool stats.
* Entering and leaving gyms is visually glitchy, and there are visual glitches during battles as well.