She's an immobile long-ranged poke/burst mage with some utility to her. She does well vs other squishy, immobile people who can't dodge her skillshots. If you get to her or she positions poorly, she's very strong. If you dive her, she dies. Seeing as Brand is exactly the sort of mage she fares well against, yeah... you got stomped. Sorry man.
She's also one of the mages who benefit the most from some of the new changes to the game, with the new masteries AND Luden's Echo being very strong on her and covering up some of her weak spots. Her flaws of high mana costs, immobility and being one of the squishiest champs in the game are still there, just easier to get around. Since you couldn't actually GET to her and she outranged you, you had a bad time. Lux is scary against teams that can be kited and she does well against teams that are so squishy she can poke them down. She sieges very well. You got countered.
That said... her ult is strong but unless you're positioned VERY poorly it's not gonna hit more than one, maybe two of your team. Her snare is the same way. This means her main teamfight skills are her E and her W. Her W is a shield... useful but not really doing any hurt. Her E is her bread and butter. If you wanna wreck Lux... engage her team HARD, and if possible, have a flanking assassin or bruiser to dive at her when she moves forward to start poking your team. Champions who can assure they reach her counter her hard come lategame teamfighting, since she has nothing even resembling an escape. Shaco, Rengar, Eve and Twitch can all slip in on her. Zed, Talon, Akali, Nidalee and Kha'zix can usually reach her and blow her up if they play smart. Malphite or Wukong can hard-engage her and force her to flee or die. Xerath or Ziggs can try to out-poke her. Yasuo can wreck her in lane. She can't handle tanky teams of people who can all-in... but if you just dance around poking each other, you're giving her what she wants. Don't do that.
Personally I think Lux is in a good spot. They need to bring more mages to her level, not nerf her. She's one of the only mages worth a crap right now and that's sad.