The NDA is lifted now it's gone closed beta so I figured I'd post a couple of things.
I've been exclusively a demo-lock so far in TBC, mostly because I'm a demo-lock on live and there's really not much PvP or isntances going on yet.
Felguard - its a great solo grinding pet. It holds aggro way better than a VW and it's DPS is very nice (I think my max cleave so far is over 700). It is however fairly fragile still, it can handle same level mobs OK but you best hope you can fear any elite mobs :p I don't see it as a PvP pet (its straight DPS after it's initial intercept), and in instances I usually end up using a succubus for seduce (I've only done the first two instances so far though). No one has raided yet in TBC but I really can't see it as a raid pet
Black Book - Has changed to also work with the felguard. I never used the black book on live server, I have it perma-equipped in TBC for solo grinding. Felguard + black book is a sight to see, I'm sure there's more efficient trinkets to use but it saves you if you get a 3-pull.
DS - there is no DS for the felguard (MD is 10% dmg as per succubus). However they have changed VW and felhunter DS. Felhunter is now +3% mana a tick and VW +2% hp a tick, i.e. they reversed the percentages. At least that's what the talent shows, I haven't actually tested it but speculation is everyone's going to have so many more hp in TBC that +3% hp would be over-powered hence the change.
Gear - I have a mix of BWL and AQ40 stuff, so far I've only replaced my wand and cloak (both with green drops). If you have MC quality gear though expect to replace most of it in the first couple of days in TBC :p +stamina is a huge stat now, it's common for green items to have +30 or more (I got a green head item with +60 stam, no other stats).
Spells - there are new ranks of a lot of our spells (I'm only 65 so not had many). Also pets get new version of their spells, my imp's blood pact now gives 1000hp, it's fire shield is 25dmg). I haven't done much with the all-new spells though, incinerate I don't like but then I'm demo spec'd.
Drain Life is finally what it should be, I get 250hp a tick back now (think it does around 190dmg), that's with fel armour and demonic aegis. New bandages I was getting over 500hp a tick heals to