Trading non-identified items usually gets you a number of shards that, when stacked to 20, make an orb that can enchant a white weapon into the rarity the orb lists. Anytime I stumble on a superior white item I hold onto it for a couple levels in case I get an orb for enchanting.
Unidentified will give you shards for Transmutation. Identified will give you shards for Alteration (re-rolls the item), and sometimes shards for Alchemy depending on what the identified affixes are. Both are very useful. You will go through a lot of Alteration orbs trying to get the right affixes on a potion.
The talent trees are intriguing, but from everything I've been reading, it's not nearly as robust as it looks.
This is simply wrong, no other way to say it. There are a ton of different possible builds. For instance, with a witch you can:
1) Summoner CI build - Stays in Witch tree mostly, except to grab the summoner nodes near the templar tree. Probably grabs dual curses
2) Summoner Tank/health build - Moves to Templar tree and starts focusing on health.
3,4,5) Cold/Lightning/Fire Crit CI - Pick a color and go for crit nodes while keeping ES high
6,7,8) Cold/Lightning/Fire Crit Tank/health - Same as above but run to Templar tree
Right there are 8 basic builds. BUT WAIT. Do you want to run Dual Totems? Dual Curses? If you're running CI do you want Ghost Reaver? What about Vaal Pact? Do you want to take some weapon nodes along the way?
There are tons of ways to build these characters. Sometimes its better to think of a class external to the in game classes, like a bow/Blood magic character. You can build that with either the Duelist, the Ranger, or the Shadow. The Shadow has a CI build option, the Duelist/Ranger have Resolute Technique options.
And of course all of this leaves out the ACTIVE skills. The possibilities that arise from different active/support skills open up tons of different options, and the way you choose your actives affect how you build your passives. If you want to run Lightning Strike with LMP you probably want some +Lightning dmg nodes, and will probably need Blood Magic at some point.