See
HERE for WoTLK news:
Key points:
1. Death Knights start at level 55. MUCH information about their new abilities.
2. ALL raids can be experienced (every boss!) at
both 10 and 25 man groupings.
3. 25 man raids will get better loot than 10 man (roughly 1 tier level)
4. No lockout or progression (i.e. you can raid same instance in both 10 and 25 man configurations in the same time period.