Every Paladin who decided to walk the road of group meat shield was a newb at some point. There are certain fundamentals that you need to develop in order to be a successful tank. These aren't in game talents or skills. While those certainly come into play, there is a wide array of potential combinations that can be used by protection paladins. I'll go into those in a later post. What I'm going to cover here are the attributes that you, as a player, need to hone in order to become the best tank you can be.
Perception: Particularly if this is your first time tanking a particular instance, this becomes very important. Before the pull, you'll need to have a solid grasp of the aggro radius of each member of your party in relation to nearby mobs. You'll need to know where the rest of your party sits with regards to health and mana, especially your healers. You'll also need to note potential LoS sites, and patrol patterns, to avoid getting overextended on a pull.
Once the pull begins, you still need to keep track of all those things, but you've also got to monitor threat and crowd control on all the mobs in the pull. You also need to constantly monitor the positioning of the mobs in relation to any AOEs, Patrols, and the raid. Your perception of how things should go, and how things are going will mold the actions of the entire group.
Planning: While most DPSers, and even healers, can get away with just showing up most of the time. Tanks need to know a lot more than just "what not to stand in". You actually need to research what you're hitting ahead of time. Particularly in Heroics and Raids. I got away with a lot in my younger days as a tank. For raid bosses, my suggestion is to go to Tankspot.com, and watch those wonderful films Ciderhelm puts together for every raid boss.
For five mans, a quick browse of the instances Wowwiki page should be enough to give you a feel for the instance. There are some trash pulls that require more intense management then simply AOE Hellstorm. In wrath, they are few and far between, but when you do hit them, if you aren't prepared, you'll face wipe after wipe. The Anub'arak skirmishers in Heroic Azjol-Nerub come to mind.
For bosses, you'll definitely need to know which ones require kiting (Xevozz), which ones require strafing(Ingvar), and which ones require jumping around like morons (keristrasza). The moment you step into an instance, nothing should really be a surprise for you, because you've already done your homework.
Pacing: This is where the planning and perception come together. As the tank, you'll be responsible for pulling, and thus setting the pace of the group's progression through the instance. Unless you have an idiot in your group, in which case they'll appoint themselves the puller, and you'll wipe a lot.
Things to keep in mind are the experience level of the group, the mana and health levels of the group, the position of nearby patrols, and the complexity of the pull. If the group is relatively inexperienced, you might need to explain what is about to happen to them. You never want to pull if the healer is OoM. As you run more with a healer, you'll get a better feel as to where they're comfortable pulling, but initially, I wouldn't recommend less than 75% mana. Also, unless you massively outgear the instance, you never want to pull multiple packs at once. This means you might need to hold off on a pull until a pat moves away.
Those are the fundamental virtues of not just paladin tanking, but tanking in general. I'll get into the paladin specifics on a later date, but these are just as applicable to a bear, or warrior, or DK.