Hurry Up and Wait
You'll find that playing Words with Friends will leave you with the impression that Scrabble games are leisurely affairs. You stare at your letters for ten minutes, sigh a few times, watch an episode of "Sister Wives," stare at the letters for another ten minutes and then maybe get hit with a stroke of inspiration that you can spell "polygamy" from a hanging "y" on the board and get a bingo. Or maybe you go to sleep and look at it the next morning.
The point is, Scrabble doesn't allow you the luxury of endless thought. Your opponent, one can assume, would probably be annoyed if you took a television break. Decisions must be made within a reasonable time frame. Get in the habit of not being able to quickly assess your rack for words, and you're on the losing end of a faster-paced Scrabble game.
On the other hand, remember that Words with Friends can also get you in the habit of checking your game at a red light and putting together a word before it turns green. Not only is this dangerous, but it'll also result in sub-par Scrabble habits (and probably sub-par word choice as well) because you don't take the time to thoughtfully construct your rack.