I respecced an alt priest of mine to Shadow today, and was struck at how well designed the priest Shadow tree is. It's a model of efficiency, with hardly any wasted space. There are three different paths in the tree, a levelling build, a pvp build, and an instance build. Each path is different enough from the others, and the point distribution works beautifully when creating each build. I think that the Shadow tree is probably the best designed talent tree in the game.
As well, take a look at the single-point talents (11, 21, 31, 41pts). I think that these talents are often critical to the sucess or failure of a tree. Shadow has Mind Flay, Vampiric Embrace, Shadowform, and Vampiric Touch. These talents are all great, and form the heart of the shadow tree, the core of what it means to be shadow. A Shadow Priest makes full use of all these talents. Having all four single-point talents being awesome is very rare.
In many ways the Priest Holy tree suffers from the exact opposite. Holy Nova, Spirit of Redemption, Lightwell, and Circle of Healing are all "gimmicks", rather than being core parts of the Holy Priest.
For paladin trees, Holy's talents are decent, but not really all that exciting. They aren't as crucial to the Holy tree in the same manner as VE or Shadowform. They are also very independent of each other. Protection has outstanding talents in Holy Shield and Avenger's Shield, but Blessing of Kings and Blessing of Sanctuary contradict each other. You can't use both at the same time. Retribution comes the closest to the Shadow ideal with Seal of Command, Sanctity Aura, and Crusader Strike. However, Repentance falls more into the gimmick spell mode.
None of the paladin trees are as well designed as Shadow though. In particular, they are missing the multiple paths through the tree for the different options. You can't really differentiate between a PvP build and a levelling build, in my opinion. Oddly enough, Retribution probably comes closest to a proper design. Its problem is that the base paladin is too far slanted in the other directions.