The thief class is not very strong compared to the trickster and even the assassin. You do get dagger mastery, thats true. And the concentrations sometimes help, but you cant rely on them. What i find is horrible about the thief is the main attack you get-slice. It is 1.6 seconds long, which is not bad but the damage part is at the end which makes it horribly unreliable and sometimes can kill you. When i used to play dop, slice was fast and did its damage and that was it. I think if you wanted to make the thief class better, slice should turn into 1.1 or 1.2 seconds long. The thief should be about speed, not a slow attack that does more damage, but rarely connects.
I think this would help make the class more viable. The extra damage from time isnt worth it as the attack damage only goes up by 15% anyway.
I also think that dagger mastery might be worthwhile if you threw in a little armor piercing. Daggers dont do much damage, so even 5 armor piercing would be cool to add.