As already leaked, the game is a 6v6 third-person hero-based shooter. Heroes include magicians, robots, creatures, humans, and more. There are currently 19 different heroes, each with different abilities and playstyles that you’d come to expect from a MOBA including ranged, healers, tanks, assassins, etc.
Players have described the game to me as being very similar to DOTA in terms of its gameplay loop and mechanics. This includes killing enemies and creeps to get the in-game currency of “souls,” which allows you to buy items to make your hero stronger. According to sources, both creeps and enemies will spawn a soul upon death, which floats up and away when they die.
They had to suspend that policy for HL: Alyx though.