I really like the Warlock talent trees. They are very well designed, and the sheer variety of viable warlock builds is amazing. This is not to say that they are perfect, as there are some mediocre talents here and there, but the top end warlock talents are really strong.
There are different points in a tree where I think you can judge how good a talent tree really is. If you stopped after spending 20, 21, 30, or 31 points in the tree, can you get a good build? So let's look at the warlock trees, and how they stack up:
20 pts - Yes, Nightfall/X builds are common
21 pts - No, Siphon Life generally isn't worth stopping at
30 pts - Yes, Shadow Mastery is a great talent
31 pts - Yes, Dark Pact is a staple levelling talent, and gets a serious look from early raiders
20 pts - Yes, Dark Pact/20 is considered the best levelling build
21 pts - Yes, I'm currently running an Shadow Mastery/Demonic Sacrifice build
30 pts - Yes, Master Demonologist is possibly the high end raiding build
31 pts - Yes, Soul Link builds are considered the best in PvP
20 pts - Yes, contains a lot of staple raiding talents
21 pts - Yes, a lot of builds, including the classic SM/Ruin
30 pts - No, I haven't seen too many builds that stop at Emberstorm
31 pts - Yes, Conflag builds exist
So what we can see is that each tree contains at least 3 of the 4 possible stopping points, leading to a large number of combinations. This gives the warlock class a tremendous variety of builds.
Now lets look at the paladin class:
20 pts - Yes, Illumination is really good
21 pts - Yes, Divine Favor is also really good
30 pts - Yes, Holy Power is also a good stopping point for healing paladins
31 pts - Yes, shockadins love Holy Shock
20 pts - No, seriously, this seems like a terrible place to stop
21 pts - Yes/No, in many ways just taking 21 points seems forced by raiding into a sub-par spec, just to get that extra Blessing
*25 pts - Yes, Reckoning is a stopping point
30 pts - No, heh, can't even imagine ending with 1H-spec
31 pts - Yes, this talent is the entire point of the Protection tree
20 pts - Yes, enough talents to complement a 31 point build
21 pts - No, Sanctity Aura isn't good enough on it's own
30 pts - Yes, Vengeance is quite good
31 pts - Yes, Repentance is a solid talent
So as we can see, Holy is in really good shape. All four stopping points are completely viable. Retribution is in decent shape as well, with 3 of 4 stopping points. Protection on the other hand has 2, and one is in an awkward position.
Add to this the fact that Spiritual Focus is considered a must-have for paladins, and the number of builds are restricted and are weighted heavily towards to the Holy Tree. Realistically, the trees need to be a bit more balanced, with Protection or Retribution combinations being more viable.
Realistically there are only four paladin builds. Holy/Retribution, Retribution/Holy, Reckoning, or Protection/Holy. And the first two are far more popular than the last two. There needs to be more, and also more differention between builds in the same tree.
In fact, I think that only having two popular builds is bad for the community. It sets up a gulf between Holy paladins and Retribution paladins. In contrast, warlocks--with their multitude of builds--seem to regard differences in build and playstyle as far less of a dividing factor than paladins do.
Edit: After thinking about it a bit more, I think I need to expand on things. It's not just that the warlock trees have better stopping points, it's that choosing one stopping point over another makes a significant difference in playstyle. For example, take a warlock 30/20/0 build. If you put the extra point in Affliction (31/20/0), you get a Dark Pact build. If you put the extra point in Demonology (30/21/0), you get the SM/DS build. These two builds play very differently to each other, and where you put that last point matters a great deal, even if the previous 50 points are identical!
In contrast, take the common Retribution/Holy build (20/0/30). Here you have a choice of of Divine Favor or Repentence. But the one you choose does not matter all that much. It does not really change how you play. The build is dominated by the previous 50 points.
To be honest, the only ending paladin talent that really makes a significant playstyle change is Holy Shield. A build without Holy Shield plays very differently than a build with Holy Shield.