As a prot paladin mained player myself, i often think about rolling another tank. Tho its not about ''aeo or single target tanking'' issiues or mana at all. You see, as paladin tank we are immobile, slow tanks. We stay in position and let them come to us. We are an unbreakable tower in some other words. But when it comes to Warrior tanking, i see that they are more mobile and they are allowed to play pvp with their prot spec unlike prot paladin pvp. Druid tank is awsome due to the non plate wearing tanking class makes raiding much easier, plus utilities that being a shapeshifter gives you both in pvp and pve. I cant say i like Death Knight but blood spec can be both viable in both pve and pvp, and also if you want to carry your guildies in 5 man heroics.
TL;DR Most of the time i have struggled with my prot paladin is mainly how dull it is. But when it comes to doing the job, my dear protadin never failed me. Im not an Altholic person, i only play 1 character at a time. So, no fancy warrior charges or def cd's or pvp rushes, nor druid shapeshifting for me yet.