Boss fights get harder via more hp and more diverse adds/aoes but i wouldnt say difficult unless your in a pug full of bads.
As far as rogues go youll use the powers your given during the intro for pretty much everything through lvl 60. Sly flourish will be replaced with duelist flurry in group dungeons (also wicked reminder is a good fill in for deft in group dungeons). Your first daily (dont even know what its called) will immediately be replaced with the aoe one for solo, and later shocking execution. lurkers assault will always be your second daily and depending on your build, the most used one, but for the most part, leveling up, sly/cloud of steel (l/r mouse), daze, lash, deft (q, e, r) youll use from 1 to 60. These are also the first skills you get.