I'd love to see how hard a mage is with this. But, alas, i'm a thief. At least I'll see how your hardcoe thief works
Actually, I'd say if anything I've made it easier to be a thief. "Sleight of Hand" can definitely give the player an edge when it comes to grabbing stuff off shelves and such undetected. You do have to be quick though, because that boost to Sneak and extra 10 feet of reaching distance only lasts 10 seconds. And it's a daily power, so for any hardcoe role-players, that means taking another shot at that dreugh helmet or something has to wait until later.
So I guess it depends on playing style.
"hardcoe Thief Life" also does away with The Tower's cheap "get into anything with a 50pt lock or below" power, which I'm sure really cramps the style of a lot of thief players.