I don't really like the usual description as "well it's an RTS where you just control one unit". Uh, not really. There is no base building, no tech tree, no peons or gatherers to control. Usually just your hero.
Really, I see the MOBA genre as an evolution of the old FFA hardcore PvP MMO games. One problem with a free for all pvp MMO was high level characters picking off lowbies, and how the game gets stale after awhile when certain high level players can control everything and new players have no chance.
The most exciting time is when the server is wiped and everyone starts a new character at level 1 and you can try to gank someone but you can't just gank newbies with your max level character because you only have a newbie yourself. Those first days after a player wipe it's more about killing mobs, farming gold, collecting equipment. PvP is careful ganks and ambushes.
Shadowbane actually had a server that was supposed to wipe every month or two to give this new game experience regularly, but I don't think they actually went through with it.
In any case, no game took it to the absolute extreme- effectively wiping the players every couple hours, so you just step in and start new for a single session- not until DOTA and related games came out.
DOTA is really just a very simple and basic multiplayer RPG with mandated teams and a set victory condition. I think it would be pretty interesting to see a game that made a more FFA form of MOBA with a week or two between resets and no set victory condition, but right now it seems every MOBA is very similar to DOTA with slight differences.