I have personally only played DotA, both WC3 and 2, but I have many friends who play both.
For a noob player, LoL will be easier.
In LoL you apparently have faster attack animations. This makes it easier to last-hit creeps and get gold, whereas in DotA you need to know your character's attack speed, and time it right to get the last hit. I've had friends talk about "Only" getting 60 last hits in the first 8 minutes, whilst in DotA that is counted as phenomenal. This is also because denying is that much easier if you attack slightly faster than your opponent, as in a battle of the animations you'll always win.
Then the creeps are apparently pushovers. You can stand there and tank them if you want at relatively low levels [From what I've been told]. In DotA, this is a bad idea. You try to tank creeps below lvl 12, and you're asking to be ganked. Even lvl 12-16 your still putting yourself at risk.
Abilities in LoL I'm told have relatively low cooldowns and mana costs. There are a number of heroes with 'innate' abilities that you can just spam, and use in place of your autoattack that's how cheap they are. In DotA, no. Early game you'll be lucky to get 4 spells off in a row without running out of mana. Late game it isn't as much of a problem, but you've got to worry about cooldowns and the fact that each spell will use at least 1/6 of your mana most of the time. Spamming spells is not an option.
On the flipside, DotA has fewer "Skillshots" than LoL, where the player aims in a direction, fires the skill, and it flies in the direction and hits stuff on the way. Most stuff in DotA is targeted, and will home to its target.
My friend who for some reason always plays support, despite being a pro AD Carry, prefers support in DotA. In LoL he complains that the ults are too weak, and not all that useful. They're decent, but the ults for support in DotA blow them out of the water. Witch Doctor as an example, with his ward of death that perfectly combos with his bouncing stun grenade.
I'm not sure what systems LoL has, however DotA has a ranked build tutor. You select the tool and it will show you the top ranked builds for your character. Select a build, and it will tell you which abilities to get in which order, change the ability description to one of when you should use them, tell you your starting items, early items, core items, late game items, situational items and anything else to help you on your way.
The community on DotA, thus far, has been quite good. I play the Australian servers 'cause, conveniently enough, I'm an Aussie, and most of the time the worst thing I've come across is someone who has no idea how to play [To the extent of not using any of their abilities and merely autoattacking creeps all game until my friend and I just started telling him what to cast, where and when. "E, me, now" - ect. There was one bunch who weren't gracious victors, however we kicked their ass next round so... yeah.
One thing that DotA has that makes the game though... The SHOPKEEPERS QUIZ. Seriously. You'll rage when you get let into a game in DotA. You'll have wanted the search to have kept searching so you could play more shopkeepers. Shopkeepers is the game, the MobA side is just there as training to learn all the items. Once you do, you will get beyond godlike sprees in the shopkeeper quiz.
No, seriously, that thing is amazing. For no reason at all it is possibly the single most entertaining thing I've played recently, even if it is only selecting which items make up a recipe.