differences between classes (minor spoilers)

Early Starter Tips (NON-Spoiler Version) (contains much of the information in the above topic)

AFAIK a pure mage character is feasible (have not played a mage), though you may want a few other skills, like Lockpick and Identify. The Survivor's Curse spell is much more effective than the mage's Withering Curse.

Kaan's Mage Guide

Playing a mage character...

For a mage I'd try a bow for a weapon. Some people try to find a knife/sword with Frost on it (unlike the Freeze spell, Frost on a weapon ignores the target's resistances, so is very effective).

Archery as a warrior - my thoughts

There was a 'way of the sniper' topic in the old forum, most of which is probably in the above topic in some form. That was the only thing resembling a survivor's guide that I recall (that poster used a survivor with a bow (dagger as a backup) with Poison Weapon).

A survivor is probably the most challenging class to play, with less of a strength damage bonus than a warrior, and less of an intelligence mana bonus than a mage. Of course you wouldn't try building a pure melee fighter or a pure magic user with a survivor, so these differences aren't necessarily disadvantages.

After finishing the game with a melee warrior on the normal difficulty level I started another with an archer survivor on hard. I'd have to play a survivor on normal or a warrior on hard to be sure, but I think more of the increased challenge came from the difficulty level than the class change. In any case, if you have played the game before judicious choice of skills and play style can make a big difference in your character's performance.

I don't know if anyone has tried a pure assassin build.

As Red Green described, you do not need to be behind a target to use Assassin's Kiss; it is just a damage bonus for knives and daggers. With a very good dagger and this skill maxed, you can pretty much get up to the max damage of a good one handed sword. The game is fairly biased towards swords when it comes to unique weapons that are always in the game, but there are at least a couple unique daggers.
I don't know if the damage bonuses from Assassin's Kiss (short blades) and the warrior's Augment Damage skill (melee weapons) stack or not.

Sneak works pretty much the same as Embrace Shadows, the Fade From Sight spell or a shadow potion (none of which I've used much). Getting too far into the light or too close to an enemy will turn you visible, as will directly targeting an opponent with an attack or pulling a lever, opening a door, etc. Sneaking and Embrace Shadows consume stamina, and will expire if you run out (Fade From Sight consumes mana and IIRC shadow potions have a maximum time limit). I would guess Embrace Shadows is either more effective than Sneak or uses less stamina (1 stamina point per second when maxed), otherwise there wouldn't be much point to the skill.
I believe you can use Telekinesis to put out torches as you are sneaking around.

Alchemy Guide (for making poison potions) - see also restoration potions

Tingalf in Ars Magicana or Finnigan in the cellar of the Ducal Inn in Versistis are good sources for traps, poison, etc.

Scorpion traps are very overpowered. The spell description says level 1 scorpions have 100 hit points, but IIRC they are actually level 100.

Save before attempting to steal. Reloading is easier to do than dealing with getting caught (though there are ways to handle that, as well).
There is also an exploit for stealing, though it may make it easy enough not to be fun anymore.
If you place a container in your inventory and open it, then drag and drop loot into the container it is not considered stealing, so you can simply walk away. It is considered stealing when you remove the items from the container, though, so make sure you are alone when you do so. If you have not joined the thieves' guild yet, the lowest weight container is a sack of grain.

The only skills that are broken are True Sight and the damage portion of Crossbow Expertise.

Assassin's Kiss - max if you are using a dagger

Pickpocket - mostly unnecessary, but some find it fun. Level 1 is required for a quest, though you may be able to find an item with this skill bonus. Level 2 is needed for a minor optional quest.

Lockpick - max, can get 2 levels in Verdistis

Evade Trap - IMO there are not enough dangerous traps in the game to make this worth the skill points.

Deadly Gift - If you are going to use scorpion traps you only need level 1, otherwise you may want level 4 or 5.

Elven Stamina - After the beginning of the game, stamina was rarely a problem with my warrior. It may help with Sneak.

Embrace Shadows - don't know; save first and compare it to Sneak

Poison Weapon - max. Even for an archer, this is better than Poison Arrow.

Identify - I would max; some prefer paying traders to identify. You can get one level from a quest in Ars Magicana.

Alchemy - at least level 3 or 4 for making poison potions; level 5 if you want. Can get 1 level in the Dark Forest.

Know Creature - Max if you are going to use elemental attack spells, so you can determine an opponent's resistances. If you are pretty much sticking to physical + poison damage you don't need it at all, IMO.

True Sight - broken (even without it, archery is very effective)

Blind - probably max; several opponents in the game use a Blind + Hell Spikes combination, which is quite effective, but you can follow up with an in-character attack

Skin of Poison - may be effective; I tend to avoid these types of skills simply because I don't want opponents hitting me, even if they get damage in the process (I know I will get hit, but in principle I want to maximize how fast and hard I hit an opponent and minimize how often and hard I get hit). If you are not playing a tank melee warrior, though, this could be a handy backup if you get surrounded.

Curse - Even level 1 can be quickly cast repeatedly on a target to significantly reduce its resistances; follow up with a poison (or other elemental) attack.

NecroShift - You can not do anything in the possessed body, other than scout around, so it is not very useful.

Trader's Tongue - somewhat useful in the beginning and mid portion of the game (especially if buying expensive items like charms or spell books), but eventually gold is fairly easy to come by.
can get a level in Rivertown and another in Verdistis

Ranger Sight - Elven Sight is much more effective. sights and weapon range
Eventually I started rejecting equipment with sight bonuses, so my sight range would not get too much larger than my screen size (the fog of war was useful for determining exactly where I had explored already).

Wisdom - Wisdom - is it worth the skill points?

Charm - don't know (probably ignore); it has rarely been mentioned in the forum. I think you may need to Curse a stronger target before being able to Charm it (at level 5, Charm can be resisted with a Spiritual Resistance of 25).

Magic Barrier - probably max (though you will eventually get lots of resistance bonuses on equipment)

Aura of Command - useful if you are going to use traps, resurrected or summoned creatures

Heaven's Gift - a relatively slow way to get a practically unlimited supply of potions, low level loot and gold; some people like it. At level 2 or 3 (don't recall) you can get regular health and mana potions, which you can then mix (with Alchemy level 1) to get a restoration potion to re-fill your mana, so with a moderate supply of mana to start with, the spell can be cast repeatedly as long as your right mouse button can hold out.

Survivors Instinct - whatever you wish. During battle it will not make much of a difference, but I found it (at level 2 or 3) handy to top off my health whenever I headed back to base to stash loot, visit traders, etc. After entering the wastelands I max the skill to use up some skill points, and it was all the healing I needed for most of the rest of the game (I used 3 minor restoration potions fighting 2 of the end bosses).

Free skills, and spell books you can buy (spoilers)