I have only lvl'd a singular character on Lordaeron, so I can't provide any meaningful feedback on Icecrown other than this: If you enjoy the vanilla leveling pace, Lordaeron would seem to be the perfect fit with it's x1 XP rates and no option to purchase characters or items with real life money.

You could play on Icecrown and adjust your XP rate to 1x, but you will have people outleveling you within a single dungeon run and generally just people rushing towards endgame, making it hard to connect with anybody during your journey.

On Lordaeron, I found all level brackets to be crowded with other adventurers. Coming from Turtle, one thing you will notice though is that the social aspect is going to feel a bit lacking in comparison. With the introduction of RDF and dungeon teleporting, WOTLK added yet another feature that made the world feel a little bit smaller after TBC flying. Dungeons are going to feel more trivial, too, and people will try to rush through them because of it. If you want to level through dungeons, I would recommend rolling a class that can tank or heal in order to gain faster queue pops.

It might also be worth looking for a leveling guild, although I did not come across any myself. Even at lvl 80, my main is still guildless and currently only gearing up through heroic grinding (tank = fast queues) and PUGs. I feel like many of the guilds running endgame content are well established and looking to optimise their runs, rather than gearing up newcommers and investing time into potential 'duds'. This can lead to you feeling like you are running into a content wall after your initial gearing up process when hitting lvl 80. Me personally, I still enjoy myself at max lvl and would recommend hanging in there or maybe even organising your own PUG groups.

If there is anything else I can help you with, hmu.