I am poor compared to some, you know why? Because the community prevents me from getting good. I join, I get killed ONCE, and I get screamed at by EVERYBODY to leave. Then, I get ganged up by four people so that they can laugh at me and spam "HAHAHAHA NOOB OWNED HAHAHA DIAF
Ask for help, play more.
Who cares if you get yelled at to leave or get ganked.
That's part of the game.
Speaking of which.
DotA is 'good' because of it's fast paced gameplay and balanced gameflow.
It's often a short session based game, but can be drawn out if you like, this appeals to a vast majority of gamers. It emphasizes teamwork over just being a one man army. Read: Takes skill to play well.
One of the things that made it such a popular map, besides its long history, is that players feel 'comfortable' with it. It's not so much of an alien concept any more.
Here's what I mean by comfort. There are other 'great' AoS style maps out there. Personas, EotA, ToB (rest in peace,) AotZ, DoExile, Age of Myths. But the problem is, for most of those maps... that you are thrown into a game not knowing what items to get, not even wanting to comprehend the incredibly long drawn out and math intensive tooltips for abilities (EotA.) Or complex terrain, or knowing where you should be going, who should be going with you, when to run.
It's simply "too much" for a simple 'player' to wrap their head around at the beginning.
DotA is, indeed, very simple, and that's what drives people to become comfortable with it. 'That' sole fact is why the player base is ever-growing.
Now a days, DotA has such an incredible player-base full of people that have been playing the game for a while, that if you throw a change in, even if it was something off the wall, they'd figure it out easily, and it'd then become part of the game's natural rythm.
That is what makes DotA great.
Not because of their heroes (because most of them are bland and uninteresting.)
Not because of the terrain (because it's ugly, yet functional.)
Because it is easy to learn, inspires competition, and has a great format to do it with.
EDIT: Having written this up, I am expecting less intelligent posts to be thrown at me with poor logic. So go for it.