Well, I've just spent a bunch of time reading the wikis for the game mechanics, because the game does a really poor job of teaching the player itself.
What I learned:
INT decreases cooldowns on spells (-1 cooldown per 2 INT)
More skill in any spell school does not reduce AP to cast (it used to in beta), all it does is give you two extra spell slots and reduce AP *penalties* for not have enough skill (ie, there is no bonus, but there is a penalty for low skill with some spells).
When you hit level 6, you start getting 2 ability points per level. And 3 when you hit level 11+
Rank 2 skills need 2 points to learn (I have a bunch of stupid level 1 skills atm. Doh...)
You aren't properly backstabbing unless the attack icon changes (would explain my rogue's poor dps). You have to stand in *exactly* the right spot, not just generally behind a mob.
If a rogue misses an attack from behind, the mob will turn to face you preventing further backstab attempts. This can also happen on a successful attack, but is more rare. Nobody knows if this is intended or not.
Some people say you get -1 AP cost on spellcasting per 5 INT, but Larian's frequent patching + no in-game info means nobody can confirm this/ info conflicts.
The game mechanics totally changed at the very end of beta, and release was totally different (why do devs do this???!). Most beta info is totally wrong.
There is currently nothing you can do to reduce the AP cost of spells it seems.
All NPCs restock their inventories every time you level up. This excludes certain single-purchase items.
And a whole bunch of other stuff. For those who were struggling as I was, it certainly isn't a bad idea to read some mechanics explanations on the web.
What I learned:
INT decreases cooldowns on spells (-1 cooldown per 2 INT)
More skill in any spell school does not reduce AP to cast (it used to in beta), all it does is give you two extra spell slots and reduce AP *penalties* for not have enough skill (ie, there is no bonus, but there is a penalty for low skill with some spells).
When you hit level 6, you start getting 2 ability points per level. And 3 when you hit level 11+
Rank 2 skills need 2 points to learn (I have a bunch of stupid level 1 skills atm. Doh...)
You aren't properly backstabbing unless the attack icon changes (would explain my rogue's poor dps). You have to stand in *exactly* the right spot, not just generally behind a mob.
If a rogue misses an attack from behind, the mob will turn to face you preventing further backstab attempts. This can also happen on a successful attack, but is more rare. Nobody knows if this is intended or not.
Some people say you get -1 AP cost on spellcasting per 5 INT, but Larian's frequent patching + no in-game info means nobody can confirm this/ info conflicts.
The game mechanics totally changed at the very end of beta, and release was totally different (why do devs do this???!). Most beta info is totally wrong.
There is currently nothing you can do to reduce the AP cost of spells it seems.
All NPCs restock their inventories every time you level up. This excludes certain single-purchase items.
And a whole bunch of other stuff. For those who were struggling as I was, it certainly isn't a bad idea to read some mechanics explanations on the web.
If monsters are kicking your butt then you should be doing something (which doesn't have to be fighting) to level up or get gear/powers first!! You also describe Rogues as useless when they are probably the one of the most overpowered classes at the moment (tactical retreat/backstabs and leech are stupid!!) 
