Holy doesn't look like a solo-friendly spec. My Holy gear is roughly one Tier higher than my Ret or Prot gear, but Holy seemed a lot slower than the other two.
Part of it is that there is a limited number of abilities to use while soloing.
Holy (2): Judgement, Holy Shock
Protection (3): Judgement, Hammer of the Righteous, Holy Shield
Retribution (3): Judgement, Crusader Strike, Diving Storm
While the 6s cooldown on Holy Shock is a lot better than the current cooldown, combat as Holy is still pretty boring. Judge, Shock, wait 6s seconds, Shock again. I suppose I could have worked in Consecration, but it seemed like a waste.
At 75, paladins get a new ability, Shield of Righteousness (slams the target for 200% of Block Value as holy damage). I suppose that can help out Holy, though it will hit for much less as Holy has no Strength or Block Value.
In my opinion, I really like the way the paladin plays when she has 3 decent offensive abilities with 6-10s cooldowns, as with the current WotLK Retribution and Protection. It's not as frantic as a rogue or warrior. It's measured, but it's not boring. You're always planning your next move, but you're not waiting for long cooldowns.
It's sort of sad that you have to wait for level 40 (Holy, Prot) or 50 (Ret) before you get your second low-cooldown offensive ability, and level 60 (Prot,Ret) before you can get your third.
Again, though, this is just soloing. I haven't tried Holy in groups yet.