That was patch 2.0 where much of the content was a broken buggy mess. Kael'Thas couldn't be done because he didn't work until 2.1
Patch 2.1 changed all of this. You couldn't use World Buffs in raids anymore, nor could you stack consumables like crazy. You either had a Flask or a Battle+Guardian Elixir. They also fixed the bosses, nerfed some like Vashj and vastly improved the Epic Gear in the game so that actually using it was worth it.
What you'll likely see people using constantly in Burning Crusade is: Flask, Food, Weapon Oil, Rumsey Rum and Potions. Hardcore players will use Potions on CD, Charges on Jewelcrafting Necks, Scrolls and Dark Runes.
It's a LOT less stressful to farm for even a full four hour wipefest in Burning Crusade then for a farm raid on current classic though if you're min/maxing consumables. A large part of this is that getting gold is a lot easier in BC then Vanilla. You can reasonably get 200-300+ g/hour in BC and there's a lot of ways guilds can help you get consumables.
There are other expenses in BC however. Expect to spend anywhere between 6k-10k gold to get your pre-raid BiS, and that's without counting getting epic flying mount.