From what my friends who have played LoL first then DotA have said, I'd agree LoL is dumbed down not streamlined, but in terms of skill rather than thought. Well, maybe thought and tactics too, however I play MOBAs casually, so I rarely play games where heavy tactics are important unless all my friends are online and I'm ranked up from lvl 3 matches to level 80 matches.
Basically, DotA is harder and less forgiving. In a single player game you can chalk this up to difficulty settings. In multiplayer all it does is lower the skillcap, and reduce the extent of differentiation you can achieve between players.
For example, in LoL apparently all hero auto-attacks are quite fast. No 0.5+ second animations. This may not sound like it makes a difference, but it really does. Last Hitting creeps is made that much easier as you can suddenly just attack at any time and get the kill. I have heard friends talk about only getting 60 CS [Creep Score, last hits on creeps] in the first ten minutes of a LoL game. Getting 60 CS in a whole match of DotA is decent. More CS means more gold, which means more items to make your life easier.
Additionally, DotA has denies. Killing your own creeps/towers so that your opponent doesn't get the gold. Denies, generally, should be a reasonable % of your CS count. I.E: For 60 CS, You'd want at least 15 denies too. My friends often have 22 minimum. That's 22 less CS the opponent has, let alone taking out a Tower through denial, removing 10 creeps worth of gold from their potential income.
Then there's the fact that creeps are apparently much less a threat in LoL than in DotA. In Dota, even if you're level six, the second you get attacked by a creep your teammates start asking WTH you are doing. Creeps can do a fair amount of damage in DotA, and until lvl 10 or so you have to worry about fighting creeps as well as fighting players - ignoring either will lead to your death. Still likely at the hands of a player, but if you run away on 1/5th health from a player into a swarm of their creeps there's no guarantee you're making it out, even if the player doesn't chase you. Lvl 10 onwards creeps become more trivial, but still aren't to be completely ignored.
Then there's the lower max mana pool and higher mana costs that are apparently in DotA. You can't spam your abilities. It is considered blasphemy to use any ability on a creep. Even if its a wide AoE high-damage that seems meant for killing creeps, you either don't level it as its not useful in a hero fight, or do level it because its useful in a team fight but don't use it as you need the mana pool. From what I've been told, in LoL its possible to basically spam abilities, and some heroes have abilities with such low mana cost and cooldown they are basically used in place of the auto-attack. There are similar heroes in DotA, but even they don't use such abilities out of team fights, as the mana drain would leave them in a less optimal position were one to occur.
I've heard there's some difference in ward mechanics too, but I never really investigated to find out. I generally leave warding to my friend who plays support as a carry, 'cause somehow he's just boss with them.
The Elder Scrolls games are an interesting case. They are dumbed down, streamlined, and anti-streamlined at the same time. Depth is removed from the game in favour of easier entry for newbies, however some annoying menues are made easier to use. The easy to use menus, however, are made IMO harder to use as a lot of the functionality is removed from them. This is especially noticeable in the PC Skyrim's interface, where the mouse barely works, and everything is in massive long lists you have to scroll through, rather than tiled for convenience, with images taking up half or more of the screen space, as opposed to useful information on the item. It really is a pain.
