What would a DotA MMO even mean? DotA (Defense of the Ancients) is a custom map (Blizzard's version of modding) for WarCraft 3: The Frozen Throne. Because it is custom content, Blizzard does not officially support the game, but I'm willing to bet that all the WC3 box sales in the past 3 years have been because of DotA. There are in-depth articles, tournaments, even songs about the game. One interesting feature is that it is constantly being updated and expanded by its makers, The DotA Allstars (TDA), specifically IceFrog. Items and heroes constantly get balanced, retooled, or added. The game boasts of over 95 heroes and over 100 items.
There are tons of mechanics which make DotA extremely fun and rewarding. The trend games have taken during the past decade has been to integrate elements of RPG games. This includes everything from item and cash acquisition in Counter-Strike to character stats in Dawn of War 2. Seeing your guns get progressively better or your units do more damage between missions is explicit progression. Not only are the players' skills and knowledge of the game increasing, but also the attributes of the mechanics themselves are becoming better. When done correctly, this is layer of depth which compounds the entire experience for the player.
DotA has what I like to call Local-scope character progression. The game starts, you progress a character, the game ends, and all explicit progression gained on that hero is lost; it goes out of scope. The next time you start the game, the hero is level 1, you have the standard amount of gold and no items. Everyone starts this way. An unfortunate side-effect of WC3 modding, but a brilliant mechanic. Not only must the player understand the low level game mechanics, but he also needs to know how to effectively progress his hero. Given his team composition and the enemy heroes, what are the best ability choices, and what items will be the most effective? I know not of any other game with this same strategic layer as deep as the one in DotA.
Another really important layer is that of team play. DotA is five versus five. You need to push towers, coordinate enemy hero kills, and complement each other's hero builds. The team that works together will almost always be the team to win (some game modes like All Random might give the team a bad composition; here you are at the mercy of the random-number generator). Heroes have implicit roles: some are pushers, gankers (killers), early-game nukers, late-game behemoths, etc. A well balanced team is paramount to success. Game modes like All Pick or any -Draft game bring team composition to the forefront at the start of every game. Don't pick 5 intellect nukers if the other team has 3 gankers. You will be waiting to respawn more often than playing.
But don't let this team play talk fool you: you personally are still responsible for the power of your individual hero. If one player is dying a lot, he is often termed a "feeder" because he is giving large amounts gold and experience points to the other team. When is it OK to be by yourself? Where is it OK to be by yourself? There are 2 enemy heroes unaccounted for--should you be pushing that tower? In team battles, you have to know when and where to engage (terrain matters), when to use abilities, whom to attack first, and when to retreat. Mind games, lures, and straight up fog-of-war trickery are common place. Here is another layer of depth.
All in all, I view DotA as a Strategy Mini-MMORPG. Obviously the team-centricity and individual responsible layers carry over (with some work) into the full-fledged MMORPG genre, but I'm curious about Local-scope progression. MMORPGs usually have global scope progression. Meaning that every time you log in, the character has retained all explicit progression from the last session. Players who level alts or different jobs/classes can make the explicit progression into a challenge--see how fast they can get to the level cap--but there is no real game layer which all players need to learn and master. Sure MMORPGs have talents and specializations, but you are pretty much locked in to your choices for the length of your character's career. There is never a time in your character's career when you need to actively think: I should first get Rank 1 of a stun spell before max'ing passive damage increase because I am against a very aggressive player with an early game hero, and the stun will allow me to escape if I need to.
There is no game encompassing ability progression in most MMORPGs. This is largely because the leveling aspect of character progression is usually against stupid AI-controlled enemies. Obviously a DotA MMORPG needs to be played primarily against other players.
Would it be possible to zone into Alterac Valley, have your character at level 1 with minimal gold and no gear? You have to kill other players and any monsters roaming around to get experience and gold. After you kill the General and win the match, you would be back at level 1. Would this work? Would local-scope be something players would pay monthly for? Or is everyone too much attached to global scope progression where time = gain and their shinies indicate their characters worth in the world?