Ever since I've begun to pay attention to Beacon of Light in the Holy Spec bag of tricks, I've discovered an annoying side effect: spam healing.
Ideally, you slap Beacon of Light on a tank just before a big fight so that you can heal not only everybody else but the tank as well. It's great in theory, but when you're running through an instance without a clue as to what's up ahead, you end up healing the tank when it's not necessary. The trash mobs may or may not do a lot of damage, and unless you know the instance -and the tank- pretty well, odds are good you're going to misjudge when you need Beacon of Light.
Result: spam heals on the tank.
Side result: Healer looks like he's wasting mana.
Part of being a good healing Paladin is actually anticipating when someone is going to need healing so that you can start the Flash of Light/Holy Light spells just before they're going to get thwacked. If you don't know the instance, you run a greater chance of misjudging when to start the healing.
A short example: The first time we went through Drak'Tharon Keep, I was juggling everybody and keeping bodies up in the air. We survived pretty well all the way up until we reached King Dred, and then the debuffs he threw down simply overwhelmed my ability to keep up. Being too close to Dred when he unleashed Bellowing Roar didn't help either, and the party ended up wiping. The next time I ran through the Keep, I anticipated what was ahead and not only kept out of range but was ready and casting the healing spells slightly before Dred unleashed his attacks.
That kind of ties in with something that I've noticed about the new dungeon finder tool. If there's a drawback to the tool, it's that the random selector will throw the noob for a loop. For the noob, every instance is new, and with the instant gratification of the tool there's no time to strategize beforehand with your group before you're off and running.
Of course, that sort of issue will crop up in regular old pickup groups as well, but you can minimize that by getting a feel for the people and the instance you'll be going through beforehand.
While the Healadin's tools are great, there's no substitute for experience and advice.