One type of Pokemon per gym, a 'decay' system to promote turnover/more effort to keep the money in the gym.
The raid bosses look to be powerful pokemon, but not actually legendaries. But you get rewarded rare berries and TMs to change movesets. That will either be a good thing or will lead to one optimized moveset for every mon.
I logged into the game and see at least three new gyms on the block towards where I park for work. Any ETA when they go live? Days, weeks?