When you ding Level 75, you should rush home to embrace your trainers. Not just because you've missed them (there's truly very little reason to bother going back for training before this level), but because this is when you learn Shield of Righteousness. This is going to be extremely handy for leveling Holy and Protection Paladins -- it's a nice ka-pow spell on a 6-second cooldown. You'll be using this a lot. You'll wonder why it took you 75 levels to learn such a badass spell that scales off a freaking shield.
At Level 75, you should be headed down to Grizzly Hills, which is oddly located right after a starting area. There are lots of quests here, particularly PvP quests which I highly recommend doing not just because I'm a PvP freak but because the quests award Venture Coins, which are used to purchase some of the best Librams for this level and a bit further on. There's the Venture Co. Libram of Mostly Holy Deeds, Venture Co. Libram of Protection, and Venture Co. Libram of Retribution which can be bought for 30 Venture Coins apiece.
If you're averse to PvP, don't worry. It's entirely possible to do all the Grizzly Hills PvP quests without actually killing (or better, getting killed by) enemy players. I know, it's kind of weird that way. If you time your questing right, you can simply pick off the relatively easy NPCs instead and risk being flagged for PvP for a short while. As a bonus, you can pick up the Grizzled Veteran Achievement to boast to your friends.
Level 76 to 77
Ok, Level 76 is a significant level because it allows you to go into Sholazar Basin. Not that anything stopped you before, but at Level 76 you get to fly there on a whirlygig. This kicks off pretty much all the quests in Sholazar Basin, which is a really good place to quest as you can do pretty much everything at the same time just by going around killing everything in sight. It's a Hemet Nesingwary zone, what did you expect?
You can level to 77 here or in the other similar level zone, Zul'drak. The difference is that you can start quests in Zul'drak as early as Level 74, whereas you'd be puttering around doing nothing in Sholazar unless you're Level 76. So even though I mentioned Grizzly Hills as a good area to quest, Zul'drak is just as good a place to level. Prior to Patch 3.2, it was especially attractive because of the significant number of undead roaming about.
Zul'drak is also special because you can pick up a significant weapon upgrade that should last you until Level 80 with the Ring of Anguish quest chain. You can actually start the chain at Level 75 by talking to a goblin in the Underbelly. Just be sure to bring some friends and do the chain at off-peak times to minimize quest conflicts with other groups on the same chain. At the end of it, Holy Paladins walk away with a Screw-Sprung Fixer-Upper, and Retribution Paladins get the De-Raged Waraxe. Protection Paladins should get the Chilly Slobberknocker, which sells for 15 Gold, 96 Silver, and 76 Copper.
Of course, it's also in Zul'drak that Protection Paladins actually get one of the few blue items specifically geared for tanks, the Clutch of the Storm Giant, which is a reward at the end of the short and fun quest chain from the caged storm giant. Holy Paladins can pick up Life-Light Pauldrons from the same chain. Retribution Paladins should do the quest chain simply because I think they'll get a kick out of slaughtering a hundred scourge and three badass undead. Between Sholazar Basin and Zul'drak, you should have enough quests to hit Level 77. If this is your first character, you should head to Dalaran and learn Cold-Weather Flying, which should cost 1,000 Gold. I told you that Chilly Slobberknocker would come in handy.
Level 78 to 80
There are two significant choices here, as Level 77 opens up two zones only accessible through flying. The first, Icecrown, assures a lot of blue item quest rewards from several short quest chains. The second, Storm Peaks, leads the way for a lengthy reputation grind to purchase the best PvE shoulder enchants. Both are excellent options and you're likely to hit Level 80 long before you finish all the quests in either zone. Both are also phased zones which need to be completed to a degree -- there will be many instances where you will be grouped with a player who hasn't opened up that phase of the zone and will be unable to interact with you even though you're in the same area.
Phasing technology is used the most in these two end zones, such as the opening of the Argent Vanguard and the Shadow Vault in Icecrown or Dun Nifelheim in Storm Peaks. Opening up these phases are certainly optional, but be prepared to withstand the ostracism (technologically and socially) as other max-level players partake of these phased zones.
Personally, my zone of choice is Icecrown. It is deeper steeped in Lich King lore, and there are a ton of 'kill gajillions of undead' quests. It also follows the travails of the game's premier Paladin, Tirion Fordring, in a pretty epic quest chain which is, again, phased. I would rattle off the blue rewards from these zones, but at this point they would be quickly replaced by gear available at the endgame. Suffice it to say that there are substantial plate rewards every step of the way.
When finally you hit Level 80, you should once again rush to your trainer (you owe her money) to learn max ranks of key spells and the first rank of the bubbly delicious Sacred Shield. This is one of the most important spells in every Paladin's arsenal. Mastery of Sacred Shield is a critical skill and is a good indicator of fairly decent Paladins. In particular, Patch 3.2 buffed Sacred Shield with a HoT component when paired with Flash of Light. Rule of thumb: always have it up. I mean, obviously, if you're just chilling in Dalaran, you shouldn't waste your mana. But during an encounter, don't forget to use it. Sacred Shield is the last spell you learn in Wrath of the Lich King, and its mastery is an indication of experience wielding the Light.
So you're 80, now what? You know the drill. The game is yours to play. Have fun, muck around... after all, when the next expansion is unleashed, there'll probably be more levels to slog through. Who knows, you might even reroll. Woof, woof.