Steep learning curve. The general consensus is that this game takes about 100 hours to get used to. From there on it could take a player hundreds more to develop "map awareness." Map awareness refers to the ability to take in myriad stimuli such as player locations, ability cooldowns, Fog of War visibility, runes (power-ups that spawn every minute). Then you have the huge cast of heroes (around 100 playable classes), all with their own set of skills. It pays to know how to play your hero as well as the enemy's. There is a lot to learn.
This huge amount of need-to-know information means that Dota 2 has a ton of potential for diversity. This helps, since a normal match can last from 40 minutes to an hour. This is a significant time investment for a one-shot. The game is addictive so repeat play can be expected.
The game's art style is a reserved take on fantasy. Player character models jump from the background, effectively communicating their importance. The background itself is a serene deciduous woodland through which runs a stream. Each team has an Ancient, home base, which takes the form of a magical shrine. Dota 2 uses Valve's Source Engine, and therefore runs well on both entry level and high end systems.
The gameplay, as with most MOBA style games (League of Legends, Heroes of Newerth), involves an isometric viewpoint (top down) and player movement controlled by mouse clicks. This style of play stems from the genre's origin as a Warcraft 3 mod. Players familiar with the RTS will feel at home. This particular style of player input allows for a great degree of positioning accuracy, which is important considering the tactical nature of the MOBA.
Plays best with friends. The game's community, like all competitive online games, can be toxic towards new players. The best thing a new player can do is try to pick useful nuggets of information out of a tirade. The best teams work to compensate for mistakes through flexible play.