This does NOT protect the target from spell damage. This should not be used as a last minute protection if the target is being hit by spells (such as anyone running from a Mage in PvP).
Unlike Divine Protection, this does not remove all preexisting debuffs (only physical like Bleed), so it will NOT double as a Blessing of Freedom.
Against rogues, if they have any stacks of deadly poison on the target you BoP, a rogue can still use Envenom to damage the target because it is a Nature damage attack, not physical (its a rare thing but it can interupt something like a bandage or be potentially lethal if you aren't careful.)
People protected by Blessing of Protection can still cast spells.
While this skill is effective in PvP against warriors, rogues, and hunters, they all have abilities that can hit through the shield. Those abilities include Kick, Thunderclap, Pummel, etc.
Players protected by Blessing of Protection lose aggro while under its duration. Tanking paladins can use this as a last resort method of aggro management, or because of a player's inability to perform melee actions through the shield, an effective way to reign in a melee dps class who is unable to control their own aggro. The threat reduction only lasts for the duration of the blessing however, so a tanking paladin needs to make sure they are able to generate enough threat to keep aggro when the blessing expires.
The paladin ability Judgement is considered to be a physical ability and cannot be used while Blessing of Protecion is active.