MOBA is a perfect hybrid mix between RTS & RPG.
MOBA is fun and addictive because:
1. You generally control 1 hero only rather than 100+ units + base management of RTS. Much easier for noobs. Both DOTA and LOL came from Warcraft 3 custom map Dota 1. Tons of people played Dota 1 rather than WC3 ladder game because of this reason. 1 hero to buff up only.
2. There is an RPG element of getting gold & exp and buying items that bring tons of different strategy for one hero. And you still have 100 other heroes to master. Tons of replay value.
3. MOBA is a teamgame. You know that satisfying feeling when you pull off an amazing teamwork in CounterStrike? Same logic and fun applies here. MOBA is generally 5v5 and requires TON of teamwork and the reward is absolutely amazing.
not a fan of tower defense, except for the Orcs Must Die games.
Tried DOTA 2 and couldn't figure out how to be successful at it (against bots). didn't give it much time though because it didn't make sense. You use a single hero who can't really do shit...can't take down defenses because you die like immediately. trying to help the bot minions or whetever seemed pointless as they died easily also.
Well technically they're all part of what Dota 1 came to be (Aeon of Strife).
Unless you are saying there were other legit MOBA games prior to AoS/Dota 1.
People shouldn't even make the tower defense comparison because most MOBAs really have no gameplay in common with TDs. The only thing similar is that you have computer controlled waves of minions following paths. That's it. There just happen to be "towers" along those paths.
They are addictive because its a competitive genre. Playing MOBA's is all about the competition, testing your skills and knowledge against others. Its also because there is so much replayability. Even though every game starts the same way (level 1 heroes, 5v5 teams, end goal is to destroy the enemy base), every match develops differently from the last because thats just the nature of the game. The huge number of heroes and items creates so many different scenarios, and the way the early parts of the game determine how the later parts of the game play out.
But really it comes down to the competition.
^You said it^ -- Replayability and the sheer competition makes it what it is.
I have the luxury of 5v5 in house games on a weekend basis. That's 10 of us crammed into my family room having a blast.
I started playing MOBA games back in 2004, I think it was, with 5.84b DOTA 1. Ever since then I've been addicted to them. I played that until Heroes of Newerth was released for beta and kept with it for a long time until the community overall turned into crap. Delved into LoL for a bit, absolutely hated it. Now I am a level 1 DOTA2 noob.
I will never put down a MOBA game again until I die, it's as simple as that.
I could see that being fun. Too bad I'm about the only MOBA player I know, in the small group I know that PC game.