Gwarble. I'd seriously suggest giving the T4 forum a search on stealth specifically, as the details are annoying and I can't remember them off the top of my head. Suffice it to say that stealth is actually
really powerful and stuff "seeing you" doesn't quite do what you might expect it to. Insofar as I can recall, just because they can see you one turn, doesn't mean they see you the next. Stealth is just... really nice, but the mechanics are somewhat obtuse.
As for DS/Flurry, I wouldn't max those early, myself. I usually get flurry to about 3/5 (around 90-100% damage per hit) and leave it for quite a while, and there's no telling if I'll invest much in DS at all (stunning's good, killing is better, and DS+dirty fighting is usually enough to kill most things even with both at 1/5). Sweep and whirlwind are both good, as well.
Traps will get you through most of the tier one stuff. Bear trap alone is enough to kill most of the melee bosses, really, if you felt like kiting a lot (obviously not as viable with a ghoul rogue [also note: ghoul rogues are pretty solid. They're beefy.].), and it makes kiting in general a sinch. Just remember you can place a trap directly below you and then step off it.
Other than that... I'unno. Don't forget that instant melee attack you've got, it's nice. You probably want to prioritize debuffing daggers (icy, elemental, slime, etc.) since you'll be doing plenty of damage anyway. The poison tree's fairly solid for that, too.
There's... probably plenty more that could be said. Check the T4 forums for character reports, I think we had a few rogue winners fairly recently. The posts on those should be somewhat illuminating. E: Actually, that was a shadowblade. Ehn. E2: Though according to the vault, tiger_eye
did win with a (really freaking weird) ghoul rogue that... as near as I can tell, went, uh.
Staves. ... man, I don't know what the hell went on with that critter.