The big blueberry, a Warlock's most lovable pet. Obtained at Level 10, the big blue is usually a Warlock's favored solo leveling companion because it can tank, having a hefty amount of health and the taunting abilities Torment and Suffering. Out of combat, it can regenerate lost health quickly with Consume Shadows, and in a pinch can be Sacrificed to grant a Warlock's version of Power Word: Shield. It isn't specialized for any tree, although a Demonologist will get the most bang for the blueberry buck.
The reliable Voidwalker won't be changing much in Wrath of the Lich King, but will be getting a few improvements with some talents from the Demonology tree such as the consolidated Fel Intellect and Fel Stamina, now called Fel Vitality, which rightfully increases Stamina as well as Intellect. Along with all demons, it also gains Avoidance, a baseline ability that helps it mitigate AoE damage by a massive 80%. Improved Voidwalker has not been changed, but a key talent that used to be perfect with the big blueberry -- Soul Link -- was reduced to a 15% damage absorption and no damage bonuses. The good news, however, is that Soul Link is now a Tier 3 talent, allowing almost any Warlock to pick it up for more survivability.
And the Voidwalker is all about survivability. With the exception of Unholy Power, all the talents in the Demonology tree that affect or require the Voidwalker are geared towards lasting longer. Demonic Sacrifice -- which will not work with Sacrifice, by the way -- restores 2% of total health every 4 seconds for 30 minutes. It's a silly talent, really, but fortunately it won't be a pre-requisite for anything in Wrath. Master Demonologist is unchanged, granting 10% damage mitigation for both the Warlock and the big blue.
The new Demonic Empowerment makes the Voidwalker a better tank and is an excellent spell for soloing content. It's even great for situations where the demon needs to off-tank something in an instance. And then there's Demonic Resilience, formerly a partner for Soul Link, which reduces damage taken by the pet by 15%. Taken together, this makes for one extremely hardy pet in the Demonology tree. Fel Synergy works even better as damage the Warlock deals heals the pet, in addition to giving it more armor, mana, and health.
While there are a few talents that increase the Voidwalker's offensive capabilities, make no mistake about its purpose... it's there to keep you alive. Even in PvP. Blue is the best demon to have against melee classes because of it's high mitigation if you have Soul Link. Make sure to key bind Sacrifice, which is the last great service a dying Voidwalker can give you... before you uninterruptably summon him again.
[EDIT: Consume Shadows confers increased stealth detection! Missed this because I hardly use blue in Beta. This means that it makes more sense now. You want a Voidwalker out against a Rogue! -- thanks Rickzor!]