I can only comment on Prot build, as I haven't done enough with the Holy tree to know it's in and out's.
The Prot tree is commonly accepted to be the AE grinder tree of the paladin spec. This is due to it's ability to grow more potent with the more mobs you have on you at any one time.
For the most part there isn't much difference from soloing in Pre-TBC areas, and Outland. A Prot pally will still try to find mobs who don't cast spells, and don't have to many stuns, knockdowns, or other effects which would slow down not only your damage potential, but also your healing potetial.
While soloing I tend to try and keep anywhere from 4-7 mobs on me at any one time, this seems to be a decent number where incoming damage and BoL/JoL balance out. With that number of mobs, you will also find Reckoning and Redoubt/Holy Shield being up more often, which leads to more shield blocks meaning more damage to everyone on top of Ret Aura and your normal sword slashes.
Unlike the Ret tree, Prot doesn't lay down huge numbers(Not counting Avenger's Shield Crits), but lays down a constant high rate low DPS count, which is why having more mobs makes the tree potentially more potent(With the above restrictions) in terms of damage and XP/hour. More mobs means more mobs effected by Consecration, shield spikes, Retu Aura, BoS shield blocks, ect and all mobs being effected about evenly across the board, so should commonly see 2-3 mobs die at one time.
The biggest bane of Prot builds are spellcasters, since armor doesn't do squat against spell damage. You will have to decide which way you want to deal with each spellcaster type seperately. Some you can just soak up the damage and not suffer to badly, others should be used with SoJ instead of SoL for those fights in order to stun as much as possible, or you can carry a Ret style set of gear including a great weapon to try and up your single target DPS high enough to take down the spellcaster a bit faster.
If I am going after any spell casters, first thing I see is how easy it would be to get some form of melee type mob in combat with me, to assist in procing Reckoning and whatnot. The Ogres of Nagrand are a good example of this being a decent tactic since the Ogre spell casters are more annoying then dangerous(at least I find them more annoying the dangerous).
Another bonus to leveling Prot is the fact that if your server is anything like mine, tanks are in demand more so then healers and far more then DPS. So you should have an easier time getting groups, on top of having a good solo potential.
It's a bit early so I might be leaving out some information, but that's Prot soloing in a nutshell.