Please start with the realization that League was created with the intention of removing mechanics that are either massively unfun to play against or just aren't all that interesting/satisfying. This is, after all, a game, and everyone should have fun playing it.
Fun for a casual and fun for a pro are two completely different things.
If you want to argue that LoL is more fun for a casual player, you would have a great argument. Its mechanics are much simpler and more streamlined, and the game is much easier to get into. However, when it comes to depth and skill, DotA wins hands down, at the cost of a much higher entry fee. DotA is the more competitive game. There are more strategies, deeper mechanics, and a much higher skill ceiling than in LoL.
I 'll concede that LoL is the better casual game, but if you want to say that LoL is the better casual AND competitive game, you're going to lose the argument.
Now in terms of certain mechanics being "un-fun", this is just an opinion. What may be "un-fun" to a casual may be extremely fun to a hardcore player. Take Starcraft 2 - a very hardcore game. To a casual, that amount of micromanagement, scouting, building, attacking, using abilities, knowing timings and build orders, counters, and race matchups for every map is NOT FUN. However, to a competitive player all these things make the game more interesting, deep, and skill-based.
Here are some mechanics in DotA that hardcore players like but which casuals find revolting:
-Denying
-Teleport Scrolls
-Mana Burn
-Magic Immunity
-Long stuns
-Gold on Death
-Costly invis counters (can't just buy oracles and be done with it)
-Tree juking
-Complex items
-Hero Attributes (STR, AGI, INT)
-Dozens of lane setups
-Buybacks
-Much more..
Here are some mechanics in LoL that casuals love:
-Flash
-Ghost
-Cleanse
-Free teleport
-No losing gold on death
-Shorter respawn times if you're feeding
-No denying
-No juking (at least it's a very nerfed version of it)
-Towers that do insane damage early on
-One-dimensional heroes
Not only does it not work effectively on the people it's primarily targeting (in Dota at least, mages get very large mana pools), it's basically a mechanism that directly says "I don't want you to play". Percentage burn sorta solves this, but then it's just a giant middle finger to everyone on the other end.
Lol this is a terrible argument. If taking all someone's mana is a mechanism that says "I don't want you to play", then what is taking all of someone's health? You're telling me that waiting to respawn for 30+ seconds is less of an "anti-fun" than mana burn? Mana burn is just one way of disabling somebody. Silence is another way of disabling somebody. In both scenarios you can't cast spells. Stuns are another way of disabling somebody. DotA has more ways to disable people, and less ways to escape disables (such as Cleanse or Quicksilver Sash). Instead, the player is expected to be pro-active and either pop magic immunity beforehand, "dodge" the disable with several items and skills, or disable your opponent first.
This is the difference in design philosophy between the two games. DotA is pro-active, LoL is re-active. It doesn't take much skill to react to a certain circumstance every game, it takes much more to prevent that circumstance from ever happening.
Again, another mechanic that is "I don't want you to play". I never quite understood why people loved mechanics that ruined enemy games more than pushed them towards a win. Though to be fair, it's not something you see a ton in your average dota pub.
Denying, especially towers, is an extremely important part of a professional or competitive DotA game. In DotA, supports do more than just sit in the lane "herp derp" spam click my spells on the enemy; they are responsible for that, but also responsible for denying and creep pulling as well. Denying makes the laning phase much more active, it gives you more things to do than click spells whenever they're off cooldown. Tower denying is another beast altogether. It gives you the opportunity, with excellent timing, to deny the enemy team a HUGE amount of gold, often at great risk to your own life if the whole enemy team is at the tower.
I get that it's a counterbalance to carries farming up expensive items, but still doesn't change that it's another "I don't want you to play" mechanic. This would be somewhat analogous to, every time you scored points in a basketball game, tying weights on the enemy players and removing some (if any) that you have on. At least the basketball equivalent of league is just being able to go faster after each point.
Once again, it's the difference between pro-active and re-active design philosophies. In DotA, if you're pro-active you can gank the enemy carries over and over until they've become a non-threat and you win the game. In LoL it's just a farm fest, and it's much less gank-oriented, since ganking somebody only slows down their farm in a minor way. In other words, you're punished more for dying in DotA because the design philosophy is: Don't die. Or if you do die, make sure the exchange was worth it. When supports and nukers die it's not that big of a deal because they don't need that much farm. However, when the carry dies, it can set them back quite a bit.
So what I'm saying is that losing gold on death opens up a whole new style of play in DotA that doesn't really exist in LoL: Ganking teams. Literally, you can have teams whose only purpose is to go around the map assassinating heroes all game. The enemy team may have a much better late game, but it never gets to late game because you keep them down so long that you win. In LoL, ganking teams really don't exist. The payoff isn't worth it, and with mechanics like Flash and towers that do massive damage, it's better to just farm for the majority of the game and participate in teamfights later.
I'm not a big fan of the boring, passive play that LoL promotes. It's great for casual players, but not people who want an exciting, action-packed game the whole way through.
I find that it has way too many stupid mechanics that are overly focused on ruining one side's time, and I also dislike balance by OPness
What is "balanced by OPness"? I've heard this before, and it doesn't make any sense to me. Do you mean that LoL's balance is so homogenized that nothing seems any better than anything else?
Also, what say you to all of the mechanics League has that Dota doesn't? Most of the more interesting skill mechanisms, something better than "click targeted nuke with stun" as a default ability, scaling mages, the class of "mark and consume" abilities, pretty sure Dota has no recover mana on hit, spell vamp as a stat, a non-obtuse on-hit system...
I'm not sure what you mean by "more interesting skill mechanisms". In terms of that, I think DotA wins hands down.
For example, what about a hero who plays something like Magicka (the indie game), that when you combine 1 of 3 elements you can make up to 10 different spells? LoL has nothing like that. What about a hero who can "steal" any spell in the game from his opponent, replicating it for his own use? LoL has nothing like that. What about a hero who gives his entire team physical damage immunity for a certain period of time? These are all really neat, game-changing mechanics; I think they are much more interesting than anything I've ever seen in LoL.
In terms of getting mana every hit...I mean that's interesting? I'm not sure how that's neat or anything, just sounds like another way to regenerate mana. Of course mana management in LoL is much easier than in DotA because your natural pool regenerates much faster, you can build runes to give you extra mana, or take Clarity, or buy cheap items (like Chalice) to make sure you never run out. But that's a whole nother can of worms I don't want to go into.
So like I said, LoL is better for casual players. It has more simple, streamlined, casual mechanics. Riot's motto is, "We can't do anything anti-fun". IceFrog's motto is, "If it improves the game, it's going in there". Most LoL players probably think a 150 gold item which makes the whole team invisible and not appear on the minimap would be a HORRIBLE mechanic, but IceFrog doesn't care because he'll do what it takes to make the game more pro-active and action-packed.
They're definitely both good games, I just think people should realize the audiences they're each targetting.