As someone who does not post on these forums very often (if ever), I felt it was my duty to the Giant Bomb community to reply to this thread. I have around 2500 games of League of Legends played, and 41 'real' games of Dota 2. I own every character in LoL with at least one skin for each character. I have also played LoL once in the past month because I've been playing Dota 2.
For me, the characters being free and accessible from the start in Dota 2 isn't a draw in itself (being that I own all of the LoL characters, including the newest one) but it certainly helped me make the switch, knowing I wouldn't have a limited pool of heroes.
Dota 2 is a harder game than LoL, it is more complex, it has a significant number of design decisions that vastly differentiate it from LoL, which as someone coming from that game, totally threw me off to start with. The differences between LoL and Dota 2 seem small on the surface, but to someone who has played a lot of either game, they make a huge difference.
You cannot spam your spells in Dota 2.
This difference is by far the biggest thing that you have to adjust to coming from LoL. Mana pools in Dota 2 are extremely restrictive at the start of the game for pretty much every character. If you waste your mana on spamming spells, your base mana regen is going to ensure that you will stay OOM for a long, long time. Mana is a resource that can be as important as Attack Speed or any other offensive stat, and if your carry is a low mana pool hero, yet has a way to get mana from his support (Keeper of the Light, Arcane Boots), the carry becomes infinitely more powerful in that they will be getting a mana bar refill constantly. The proper usage of abilities is a lot more important in Dota than in LoL unless you have a way to regain lost mana very quickly. Using a TP scroll even costs you some mana, so if you are a low mana pool hero, and you've used all your mana, you're not TPing anywhere.
The map is bigger in Dota 2, and every single character can teleport for a price.
This change was huge to me. Having to commit to carrying a Teleport Scroll at almost all times and being aware of things that are happening across the other side of the map just isn't something that you have to consider in LoL at all. LoL has the small few characters that can teleport across the map, but otherwise it's restricted to a summoner spell that few people take. Dota 2 map awareness is absolutely huge for this reason and if the other team is taking good advantage of their TP Scrolls at good times, you will constantly feel pressured in your lane. In the early game, 135 gold for a TP scroll can seem like (and is) a lot of money, especially for a support character or someone who is new and cannot last hit very well, but this just emphasises how important good use of a TP scroll is.
The differences in heroes and not knowing what they are capable of.
As someone coming from LoL, do not take the fact that you don't know what an enemy hero does lightly. Mistakes are much more aggressively punished in Dota 2 than they are in LoL. At first, if you do not know what a character can do, it's probably safe to assume they can kill you quite easily if you get out of position if they know what they are doing.
Last hitting and denying.
Last hitting feels very different in Dota 2, although at its core, it is the same concept as in LoL. Denying, however, is not in LoL at all and is quite important in the early game in Dota 2. Denying is where you attack your own creeps, effectively getting a last hit on them. It denies the enemy gold and also experience from that creep, so doing it effectively can give you an early level advantage in lane.
Summoner spells and abilities.
These just straight up don't exist in Dota 2. You have TP scrolls for a teleport (which will take up a slot in your inventory) but that's as far as it goes. Some characters will have abilities mapped to your usual Summoner Spell buttons from LoL, as they will have more than 4 abilities. LoL has the sense of abilities being very active and usable often, whereas in Dota 2, there are characters that only have a couple of abilities that you can activate. Dota 2's more flexible ability set design, leads to a very different playstyle for different characters and allows for greater differentiation in character types.
Flash gets its own section because the non-existence of this single summoner spell in Dota 2 is absolutely huge to every single League of Legends player. The fact of the matter is, for better or for worse, LoL is balanced around the existence of Flash. There have been multiple attempts by the LoL community to have it removed or altered, reasoning that it is an unnecessary crutch that allows someone to play poorly and get away with it. In its current form, Flash allows players to mistakes, have poor positioning, be completely out of mana in the middle of their lane, and still have a way to get to safety. Flash is a huge part of LoL balance now and any major alteration (or even removal) of the summoner spell would mean virtually every single character would have to be rebalanced. Riot simply cannot just 'remove Flash' without massive changes to their game, and the very existence of it (and the fact that is isn't in Dota 2) already allows Dota 2 to be in a much stronger position for competitive balance. As someone who plays mostly support in LoL, having no items and no escapes, I know how important Flash is to the balance of the game right now. Unfortunately, it is always going to be a major component in LoL balance until it is drastically changed (along with tons of champion balancing), but there are no real solid ways that they can easily balance it without altering major parts of their game.
Wards in Dota 2 are in limited supply in the shop. If you have someone on your team who buys all the wards and they are in the bottom lane, you're not going to have any wards for top lane unless they come to ward for you, or you wait until they are back in stock. I'm still undecided on whether I like this change or not, but it places more importance on having wards (for everybody, not just supports), and makes map vision a limited resource which should (rightfully) be thought of as important by any player of either game. In LoL, by the end game, you can cover the map with wards and be able to see all of the major pathing areas, but limited ward stock ensures that is never going to happen in Dota 2 at any point in the game. Whether I like this change or not, it is definitely something to consider when moving from LoL to Dota 2.
Runes (not those ones).
Runes spawn every 2 minutes in either side of the river in Dota 2. They are randomised and can be such things as Regeneration (which restores all of your HP and MP), Double Damage, or Invisibility. As someone coming from LoL, I found it incredibly hard to come to terms with the fact that anyone could potentially be invisible at any point in the game. Rune control is a big deal in the laning phase in Dota 2, especially for the mid lane. Most mid laners buy a Bottle, which is similar to the Crystalline Flask in LoL. The bottle not only refills 3 charges when you go back to base, it also refills if you pick up a Rune while holding the Bottle. Not only that, but the Rune can be stored in the Bottle and then used at the most opportune time (otherwise they are activated on pickup).
Runes (yes, those ones) and Masteries.
The lack of runes and masteries allows for a fair balanced game in my opinion. There is no hidden power, what you see is what you get in Dota 2 and quickly checking a character's items tells you what they can do. You don't have to worry about whether the enemy has hidden lifesteal from their masteries, or straight Attack Damage runes. It theoretically allows a new player to be on the same balance level as someone who has played thousands of games. If you are Summoner Level 30 against a level 1 in LoL, you have 29 Mastery points over the Level 1 guy, and most likely a full rune page, making the game unfair right from champion spawning.
The fact that items are different in Dota 2 should come as no surprise. What might be a surrpise, though, is what some of these items can do. There are items that make your character invisible, silence, stun, none of which you would ever find in LoL. These items are always a major investment for the character that buys them, but they can be incredible useful for rounding out a character and giving them much needed escapes or CC or ganking potential. For me, it's a really refreshing change from the items in LoL, and although it is hard to get used to at first, I do enjoy the more varied itemisation.
For better or for worse, there is no option to surrender in Dota 2.
I think I've covered most of the major points that differentiate the two games, but I cannot emphasise enough that after playing so many games of LoL small changes make a big difference. The differences, big or small, between the two games allow for very different playstyles within the same genre of videogame, both of which I believe are good games worth playing.
For me, the main difference between LoL and Dota 2 is one that cannot really be quantified for anybody else who hasn't been so invested in LoL. I feel like I'm learning new things all the time in every game of Dota 2 I play, which just simply is not the case in LoL anymore. Dota 2 is a refreshing change from the constant meta of LoL, with the same old characters, played in the same roles all the time. The versatility of characters and roles in Dota 2 cannot be understated as someone who is sick of the same old meta in LoL. The greater number of tools at any character's disposal in Dota 2 means that unexpected things can be achieved and that keeps me thinking about the possibilities in each new game I play- something that LoL hasn't done for me for a long, long time