You'll want to start downloading Dota 2 right now. It's available for free on Steam. Patches (which happen frequently) automatically download in the background, and there's no significant downtime during patches. You can easily play with your Steam friends, and it's easy to meet new people.
You get access to every Hero for free. You can buy cosmetics to enhance the look and feel of a hero, but spending money won't get you ahead.
You don't need to setup runes, masteries, or summoner spells before the match begins. Everyone starts on a level playing field and all decisions are made in game.
Roles in Dota are flexible. Your role may change depending on how much farm you get in the match.
Dota doesn't link player accounts to regions. This means you can play with your friends anywhere in the world. You can choose to play only on game servers in specific regions (ex. US East or US West) to reduce latency.
You need to opt-in see your profile and statistics on Dotabuff. Open the Dota 2 client, click the Settings cog (upper left), then the Options tab, click "Advanced Options" on the bottom and make sure "Expose Public Match Data" is checked. Learn More
Dota doesn't have Summoner spells like Flash. A very common item to purchase is Blink Dagger, which provides you with more mobility than Flash on a much shorter cooldown. Fewer heroes have mobility built in and instead purchase it.
There often isn't a dedicated jungler. Anyone could farm the jungle depending on the game. Creeps can tank it. Supports can stack additional jungle camps by pulling them out of the spawn point near their reset timer (every odd minute starting at 01:00). Carries can kill stacked camps for massive amounts of gold and experience.
Movement in Dota is slightly different because turning takes time. Clicking to move or attack will cause your hero to rotate to face the cast point, then move or attack only once it's facing that direction. This will take some getting used to and adds another level of depth to gameplay. Last hitting and kiting are more difficult.
Dota has three places to buy items: the main shop in your base, side shops in the top and bottom lanes, and the secret shop: a place in the jungle that sells special items you can't get elsewhere. Each team can use a Animal Courier to pick up and deliver items from these shops to your hero anywhere on the map.
Instead of Baron, Dota has Roshan, who gives gold and drops Aegis of the Immortal with no team buff. Dota doesn't have a Dragon, but does have two Runes: powerful temporary buffs that spawn in the river every 2 minutes. Barracks (inhibitors) do not respawn, and only the melee one will regen health if not destroyed.
Start with Purge's Dota 2 Guide: Welcome to Dota, You Suck. Purge's guide goes into just the right level of detail on nearly every aspect of the game. Purge also produces great videos for newer players.