Shield Block currently on Live:
5 sec cooldown
Increases chance to block by 75% for 6 sec, but will only block 2 attacks (talented)
Shield Block currently in Beta:
40 sec cooldown (talented)
Increases your chance to block and block value by 100% for 5 sec.
At its core, the current (live) shield block is based around avoiding crushing blows. A little extra mitigation on two incoming attacks is a nice benefit.
Looking at the proposed Shield Block, the cooldown is much longer even when talented. However, there are no charges to the number of attacks that can be blocked while it is up. Not only does it double SBV (for big burst threat gen with Shield Slam), but with the implementation of critical blocks, it would be possible to have SBVx4 damage reductions. Given that this damage reduction is not affected by Silence effects it will be especially useful for fights where healers go through periodic periods of being unable to heal.
At the brink of danger I stand. Clad in armour, shield and sword in hand.