Free to play, whether you like it or not.
There is a huge learning curve if you haven't played a game like Dota before, as I can attest to. Learning what every hero and item does, which runes do what, which seconds to stack or pull, and where to place wards are just some of the growing pains. Still, once you've memorized these tidbits the gameplay isn't that challenging relative to other competitive games and genres. It doesn't take the twitch reflexes of a FPS or the multitasking of a RTS to play most of the hero pool well.
I find Dota to be an almost masochistic experience. Winning or losing feels like the result of a thousand little papercuts. Those creeps you missed easy last hits on might rear their ugly little heads when the opposing Gyrocopter casually walks up to you and rock barrages you down for a first blood. For want of 450 gold a boot was lost. For want of a boot first blood was given. And so on. Games can be excruciatingly long, I would say the average length of a game is about 40 minutes. There are times when both teams do nothing for five minutes at a time, having a staredown in midlane posturing over a tier 1 tower.
So why play this borefest? It's going to sound cliche but the game is as fun or boring as you want to make it. You can pick Phantom Lancer and try to AFK farm the networth of Bill Gates if that floats your boat or you can roam around the map snowballing everyone with Tuskarr.
The free to play model is a bit of a mixed bag. On one hand, it's nice to find relatively quick games in matchmaking with the large (think 200k+ people online) pool of players. But, I can't help but feel that the community would be more tolerable if Valve charged even just a cent for the game because of who most often gets barred due to credit card requirements (minors). Still, there's no getting around the model they've chosen and it's been handled well relative to other free to play games. Dota 2 is not pay to win. There are no heroes you have to purchase. Valve have reneged and backtracked on some of their earlier promises however. Dota 2 launched without achieving parity with the Dota 1 hero pool. Some of their recent cosmetics don't follow their own guidelines (color palettes, good taste). And there are definitely experiences you miss out on if you elect out on if you elect not to spend money, though you always have the option of receiving gifts or spending countless hours farming items to trade. Take compendium matchmaking, where players enter the scenario of a game played between two professional teams. This matchmaking mode is restricted to people who have activated compendiums ($10 in the dota 2 store). Tournaments outside of Valve's own The International also require the activation of a pass that can again, be either acquired from trading or from payment.
VS. Heroes of Newerth and League of Legends. I'll be speaking from my very limited experiences playing both of the aforementioned games so take it with a pinch of salt. HoN is extremely similar to Dota 2, probably because a lot of their heroes and items are actually based off of Dota (Magebane vs. Anti-Mage for example). People who have played HoN will have no trouble transitioning into Dota or vice-versa such is their similarity. HoN and LoL both feel like they have quicker gameplay than Dota. HoN with its faster casting animations and LoL with their shorter cooldown spells. As for LoL, I actually have trouble making too many direct comparisons because they actually feel like entirely different games. No denies, creeps that give massive buffs, towers that do more damage, no courier, two abilities that aren't part of your hero's skillset, buildings that respawn, etc.
Pessimism aside, what you get from Dota 2 is: a well balanced game as of 6.78, a matchmaking system that more or less puts you at where you deserve to be, a non-intrusive free to play model, a competitive or clowny 5v5 experience.
Graphics - 7.5 - Source Engine. Meh. Every once in a while one of their weekly patches creates framerate issues. Very scaleable though, you can pretty much run it on a toaster.
Audio - 9 - The voice actors and actresses are actually simply superb. Every hero sounds unique and have a suitable voice to match. Some of the best moments are hearing how they interact with certain heroes (Tidehunter - Kunkka) or items (Timbersaw - Blink Dagger).
Gameplay - 9 - The meat and potatoes of this game. If you go on the game's dev forums there are dozens of [major] bugs that have been known to exist and still not fixed for over a year.
Multiplayer - 6 - Experiences vary from game to game on the quality of the players you get in your games. Servers can also periodically be very unstable and laggy. I'm against the general trend of companies catering to casuals (yes I went there). When I first entered the beta, other player profiles showed how many losses they had. Now it's just the wins. In the early stages of the beta, you could see your MMR with a console command (long since disabled). Valve after the launch of an external ELO/MMR service (Dotabuff's DBR), destroyed the accuracy of their metric by adding and turning on by default a privacy setting that bars the collection of specific matchmaking stats.