Impressions of a newcomer
Beyond the competitive appeal, the biggest draw of the game is the hero roster. However, this is also the game's biggest misstep. As of now, I think that there are like 102 characters that you can choose to play. Having played several already, I find that this is a case of quantity over quality. Many heroes are simply not competitive against others and would be poor choices in most matchups. Ranged characters seem to have advantages in killing creeps, destroying structures, and targeting melee characters, which can be quite frustrating and feels a bit unbalanced. I would have preferred that Valve include about 40-50 heroes in all, but make them all interesting and competitive. Instead, they have simply chosen to spam-release characters, with many being mediocre or very similar to each other. The bigger problem is that in order to become really good at the game, you'll need to study nearly all of the characters, or at least the popular ones. You'll have to know what each hero does, so that you can counter or exploit it. This drives up the high learning curve even further. Moreover, games are won or loss based on how your team interacts and your knowledge of each character's strengths and weaknesses. You almost have to have a sixth sense about when to attack as a pack, when to run, when to go it alone (such as farming a lane), and when to do an all-out pursuit of a fleeing enemy hero (which might actually get you killed in a "ganking" or by a tower). This sort of teamwork adds to the fun and intensity of the game, but also raises the stakes regarding each individual's strategic role and gameplay skills. Many of the hissy-fits and verbal abuse seen in online games has to do with players dying (or "feeding"), which is quite easy to do and often inevitable. In sum, the game places a premium on knowledge and skillful use of it.
The excessive number of characters somewhat masks the lack of diversity when it comes to battlefields. There is only one map, and it is geared toward 5v5 fights. This might be okay until you settle on a favorite roster of heroes and become good with them. After that, I can imagine that this game will get quite repetitious to all but hard-core fans. I would have much preferred that some of the time Valve spent creating heroes was instead used for making maps. Why not have some 2v2, 3v3, 4v4 maps with different scenary, environments, lanes, creeps, etc.? This is, of course, a matter of taste, but I was getting tired of the same map after just one evening of play. Again, the different configurations of teams that you can have will help keep the game from becoming boring too quickly, but there is still no good excuse not to have more maps.
The game has a lot of customization choices, beyond experimenting with different character builds and equipment. It is primarily made for multiplayer and has a good suite of options. You can play competitive against other humans, co-op against bots, have private sessions with password protection, create lobbies for public sessions, and so forth. Dota--and I think MOBA games in general--has a notoriously crude and immature community, so either be prepared for abuse or identify a group of friends to play with. The game is really not "noob" friendly, and the steep learning curve does not help. Fortunately, there is a pretty decent tutorial that comes with the game, and the bots can be a fun challenge in matches against the AI (at least as a newcomer I'm finding them challenging on medium difficulty). Since the game is free to play, it is literally open to any and everybody, which means that any and everybody will be showing up to play. I am personally a bit too old and preoccupied with real-life issues to deal with the attitudes that you find online, so I tend to play only against the AI. However, the game is definitely geared toward online play, which is probably most rewarding if you can get together a circle of friendly players.
On other issues, the game looks decent by the genre's standards. Nothing special, but it's functional. Audio is quite good, with each hero having a number of sound-bites. The game ran well on my aging computer. Valve seems to be dedicated to supporting the game with patches and content, and, of course, you can buy cosmetic items for your characters that don't impact gameplay. The game had an extremely long beta period, so many of the bugs have been ironed out. I still think that there are some serious balance issues involving the various heroes, but they are not likely to be all fixed due to the sheer number of characters and the various roles and configurations of teams that are allowed.
If you are curious at all about the game or the MOBA genre, Dota 2 is worth a try especially since it's free. If you are completely new, I highly recommend playing through the tutorial and some AI matches. Again, the game still needs some balancing tweaks, and I really do hope that more maps are added (a map editor would be great). It's good fun for a while, but skewed toward committed, hard-core players who have the time and patience to learn its many layers and complexities.