since nowadays MOBA are having a lot of appeal in internet, I decided to start playing one

I tried LoL, but I disliked the fantasy-theme it was based on

That said, here's my question: do you know any MOBA as good as LoL but not characterized by fantasy-themes? (by fantasy-themes I mean dragons--colored races/people--unoriginality--ecc.)

I've never tried it before, but i've heard of Infinite Crisis, not sure if LoL quality, but it's based off of DC comics

This list should help you.

http://www.mmobomb.com/games/moba

If you want something that has similar gameplay to LoL, just go with Dota 2. It was the first one after all.

Dota 2 can a really hard game to play sometimes though. My brother has been trying to look for a good free MMO too, but he decided to just stick with World of Warcraft.
Most MMOs are fantasy themed nowadays, but I'm sure you can find one that's "realistic" or something.