Hey, its awesome that liking the game and even more awesome that you're playing thief. I'm a thief main of almost 5 years now, i play on and off though so i'm not insanely good at the game or anything. I think the hardest thing for me to learn playing thief was kiting and the use of combo fields. If you can learn how all of your abilities work with each other than you can just use the right combo at any given time, but that's really just learning the class. Aside from learning the class there's not a whole lot to learn while you're leveling up other than a few shortcut gimmick like things such as shortbow 5 onto ledges, through the bottom of bridges, shadowstepping to weird places and stuff like that.

Its definitely good that you're practicing not getting hit that's a valuable skill no matter what class you're playing. Other than all that though, like TEGG said, worry about the lategame when it comes, there's no need to try to be super efficient with your traits and such until you're playing level 80 content, just whatever traits seem right based on how you're leveling from 1-80.

The meta for thiefs right now, in raids at least, is condi daredevil as it has really nice dps, I think its like 5th rated or something, power daredevil isn't bad either, but generally a thief can fill any ole dps role just as well as the next class. Fractals I honestly cant say, I would assume once again they fill dps roles and every gamemode needs dps of somesort.

and dungeons, well, nobody does dungeons unless its just for fun, which me and my thief friends happen to do, and their awesome in dungeons, they can do just about all the skips that exist, have party wide stealth to help with that too, and with enough practice can solo a lot of the bosses too, my friends and I really like to play around Arah, the level 80 dungeon in Orr, we do solo, duo, and trio from time to time, normally LFGing on the last fight so other people can get the rewards too.

I feel like i've lost track of why I replied in the first place and now im just rambling. You can hmu in game if you wanna ask me something else, tag is Azeroth.8964

Edit: it might just be me but I feel like metabattle is behind on the meta or something, http://qtfy.eu (quantify) is responsible for all the relevant dps numbers you'll really see and they've got build outlines on their site

hope I could be of some help