IMO, it is better to wait until you have Hammer of the Righteous before respeccing to Prot.
Going Prot Pre-HotR is a waste, that single talent alone accounts for quite a large amount of your DPS, without it, your only attacks are Judgment and your Shield, and Avenger's if it was up when you pulled.
Once you get HotR, though, you pretty much have everything in the Prot's repertoire, and can get to it. I find that Prot is very easy to level, albeit a little slower. Much, much more survivability and it is much harder to die unless you take on something you've no business trying to fight yet (usually in the case of elites). An elite your own level is generally safe, except for Group3 quests, some of those are very difficult if not outright impossible (I'm pretty sure Overlord is still impossible to solo at the appropriate level). Most Group2 quests are easily solo'd.
Thanks to BoSanc, you will never run out of Mana unless you're blowing heals like crazy (usually a sign you bit off more than you can chew), don't pass up the Avoidance gear while you quest, every Block, Dodge, and Parry will give you mana (and of course, make you take less damage), which is the whole point of being a Prot Paladin in the first place.
Defense Rating doesn't mean much while you are solo grinding; save that for dungeons and go for Avoidance and STR while leveling. When you get to 78-80, _that_ is when you wanna make sure you get up to 540 def.