For ALL characters, max out skill points in Coercion, which is basically your Persuade option in dialogue. Requires 12 cunning for the 2nd pt, 14 for the 3rd and 16 for the 4th. It is always useful. Always max out Combat Training for Warriors and Rogues as each point unlocks 2nd/3rd/4th tier talents. Only one character needs to max out Herbalism, one character for Poison-Making, and Trap-making is generally useless. Any extra points can go into Survival or Combat Tactics. Having one character max out stealing can be fun but it's usually better if it's on you for certain quest options.
Honestly, for all warrior and rogues, it doesn't matter too much what talents you pick because you'll pick up the whole tree by the end (just keep in mind the stat requirements for them). You'll reach level 20-23 by the end and can buy several extra talent points, and each tree like the Sword/Shield tree have 12 talents to buy, so you'll easily get all of them and have points left over for the general tree at the top.
For mages it's a little trickier because they have a lot more options. I've never had the courage to do something wacky like no fire/ice/lightning spells and do all glyphs or curses or whatever. Cone of Cold is the most broken spell in the game in that it freezes everything, including bosses, despite immunity, so it's up to you if you want to go that route (if it'll keep you playing when you would quit otherwise, go ahead). At least on consoles, the big AoE spells Blizzard/Tempest/Inferno could be targeted outside of my visual range through several walls, which made them pretty broken too.
Blood Magic is a specialization. Specializations are unlocked through various story options, some with party members, and others can be bought in stores. Once unlocked they are available to all characters you make of that class, there are 4 for each class. You get a specialization point at level 7 and another at 14. Blood Magic (and Reaver) require evil options in the game though, so if you don't feel like being evil you can unlock the specialization then reload a save and do it again the good way.
