I think the only one that is really bad is Speech since it doesn't help or change a lot. Blacksmith and Sagecrafting is good if you want to make awesome gear yourself. Mercentile is good if you want lots of monies.
You are now breathing manually. Lets put a little lead in the air, and see what falls over.
I like to put alot of skills in detect hidden and mercantile, I do this for the first 10 levels or so then respec out of mercantile... probably not nessacary but then I start the game with a lot of gold and never look back
Detect Hidden is a must for all builds. I suck at Dispelling (don't judge me!), so I use it and I also like Persuasion because you get some different items, can avoid some annoying fights and you get bits of unique dialogue.
Crafting (Smithing, Sagecraft and Alchemy) are amazing and break the game. Mercantile isn't half bad just to get a little gold from stuff you destroy out of the Junk category, even though money becomes a joke with a quickness. Lockpicking should be taken out of the game. Stealth doesn't seem to matter if you have one point or ten.
If you are talking about combat abilities, Rite of Flame in the Sorcery tree seems extremely over-powered to me. That spell alone is the reason I can defeat enemies so easily at my level, without it Sorcery / Finesse would be much more challenging.
It occurs low in the Sorcery tree (tier 2) so anyone can get it. 5 points in that skill allows you to instantly set up to 5 targets in a large cone in front of you on Fire, then detonate them all. The spell's range is large as well. This costs negligable amounts of mana and can be repeated every 2 seconds. One detonation deals 300 damage to each target which is WAY MORE than any other ability I have can do.