Ive got 450 hours logged on Dota and I'd have to say the thing that keeps me coming back is it's depth, variety and balance. Valve has done a superb job in creating a supremely deep game with great balance and polishing it constantly with constant patches and updates. Every game is different despite it being the same map because of the 1000s of possible combinations of heroes (over 90 to choose from) and the different stategies you need to employ to win depending on your team makeup.
On top of the fact that I got the game for free and it doesn't cost much to buy outright and its very easy to get a free betakey i think it represents phenomenal value. Sorry to sound like a fanboy but I bloody love it. its like crack for me!