I must disagree, playing a Thief myself and it’s fun to play.
Spamming Heartseeker is sometimes okay (if the enemy is at low health or you want to close in and doesn’t have another step-power ready), but most of the time I use stealth to get around the enemy and backstabbing … which is really fun imho, as you always have to move around etc.
I do much more damage this way than staying static and spamming Heartseeker.
What I dislike are that the thief doesn’t have a lot of disabling abilities … at least not such that lasts long enough to be worthwhile. Some crippling effects, but most skills only last for a sec or so