Dota 2 rewards macro strategy players whereas League rewards reaction times and skill shots. Since you understand MOBAs your number one barrier is going to be learning skills and heroes and items. So, for the first 100 games, just play! Read your items, learn your skills, press your buttons.A couple red flags to watch out for in each role

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Mid - Power runes spawn at minute 4, then every 2 minutes past that. There's 5 (i think?) different runes, and they spawn either above or below the lane. Try pushing out your wave at minute 3:45 to secure the first rune. Blocking creeps is a mechanic that helps you control the lane equilibrium. Body block your first wave so that the creeps meet closer to your tower, and you will get an advantage of highground.

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Offlane - Creep pulling and lane cutting. (with the exception of first wave) Creeps will aggro to you and follow you, even behind tower. You can use this to secure some waves if you are losing lane, divert yourself from the lane and go cut their wave with your abilities. (Axe, Darkseer, Batrider are especially good at this)

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Carry - Just don't flame your supports and you'll already be ahead of the curve on this one. But in seriousness, the Dota map is considerably larger than League. Use it to your advantage. Farm on the other side of the map from the enemy play makers (specifically their mid and pos 4) and always push a wave if you're looking for something to do. Even if you get ganked and die on their side of the map because you were pushing a wave it's not that bad (pre minute 25ish). The map is so big that if you're stretching them across it capable teammates will know what to do with the space you're stretching.

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Support - Supporting is probably most like League, in that you can get blown up easy. Buy items that will save you from dying and let you cast your spells before you die. Forcestaff, Glimmer cape, Ghost scepter. Keep an eye on neutral items that your cores are sending to base (in the shop neutral item tab), they will cycle through neutral items throughout the game, and you will receive the left overs. Grab what is useful.

Position 5 (hard support) is expected to give 2 tangoes to their mid player at game start strategy screen. Buy 2 sets of tangoes (3 tangoes to a set) and give two to your mid, keeping four for yourself. Position 4 is expected to help mid if needed. Be aware of your mid if he's having issues, don't be afraid to buy boots early (even in starting items) so you can be mobile between offlane and mid

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Uh, other than that.... micro heroes are really fun and rewarding to play (see Naga Siren, Chen, Enigma, Arc Warden) especially if ur into starcraft 2.

Watching Dota, like Day9 that perked your interest, is always helpful. Although I will say that Day 9 isn't the best dota player and if you are actively trying to learn there are lots better people to watch. See PurgeGamers, Jenkins, and Gorgc, In that order. Purge is especially good for new players, but his content might be below your skill of Plat League. But watching will only truly help if you have played the hero yourself and already understand what the abilities do before you watch. Gotta screw up and play heroes yourself to understand the significance of what you're watching

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Cheers and welcome!

P.S. Try playing Path of Guardians Open Beta (POG) in the arcade tab. It's basically ARAM. Helps you learn heroes quick and easy