At what content level does tanking class matter?

I'll be rolling a tank soon on Outland, and in the past, I've heard that druids in TBC are great OTs, but not necessarily great MTs at higher content levels.

I've played and enjoyed a druid tank in other expacs, but I'm not opposed to trying a new class (even if it's more gear dependent, and takes longer to level).

And I know that any class can handle 5 mans and Kara...but at what point do guilds start to reserve tanking slots for plate wearers? I don't ever see myself getting all the way to Sunwell, so if it's that late in the game, I'll just stick with a bear. But where do you start to see a separation, for where it's a disadvantage to have a druid as MT? If it's early enough in progression, it will influence my class choice.

As an oldschool player if i remember right, T5 + Main Tank is preferable to be a Warrior, Of Tank Druid Or Palatank and AoE Tanking For sure is palatank.
I have never played tank really but Imo you should go palatank, They can go pretty much everywhere and honnestly i think the only spot where its going to absolutely need to be a warrior and not a paladin is on illidian. I personally prefer warrior tanks, i dont know why but as a healer i feel safe when our MT is a warrior but if you want to go allAround maybe do some OS healing paladins a really cool.

So to make it simple
Best 1Target tank: Warrior
Best Of Tank: Paladin

This is for sure in my opinions and according to my experience, maybe i am missing something.

(Sorry for bad english)

Yeah Druid tank is legit completely fine even for MT, had one from t4 whole way into t6 and made it to brutallus before we quit on retail. Pally tank is great aoe tank ofc and make a great OT and druid can too, but with the gear druid is FINE to tank any given you have the gear and know what you are doing

In TBC, each tanking class has very pronounced strengths and weaknesses. Ferals have by far the highest effective health and offthreat and that is relevant everywhere from Karazhan to Sunwell. For example, other tanking classes face extreme difficulty in maintaining #2 threat position on Gruul and Lady Sacrolash (Eredar Twins) alike so feral is easily optimal in those roles. Conversely, one of the big vulnerabilities of feral is inability to avoid crushing blows which makes them less favourable in most maintanking jobs but when crushing blows have been disabled (Brutallus) or warriors and pallies would also be taking them (Fathom Lord Karathress in final phase in which he receives stun ability), ferals again compare favourably to the other two classes.

In other words, there isn't any point where guilds start favouring plate classes. From High King Maulgar to Kil'Jaeden, every guild ideally wants to have one of each tanking class. Typically the "main tank" job tends to fall to the warrior, but that doesn't make feral any less valuable.

It matters as soon as tanking begins.

Pally tanks have good AoE threat for Kara trash and Heroics, but they often get mauled on Prince or on some Heroics where the trash hits too hard. Warriors have a tough time keeping up AoE threat, but a warrior will not get destroyed by Prince P2 because they have both the mitigation and the avoidance for it. Palas lack mitigation and bears lack the avoidance to become uncrushable. Warriors are also the best choice for Nightbane because they can negate the AoE fear by simply using a Berserker Rage macro every time when Nightbane starts casting it. From my experience, even with Fear Ward and Tremor Totem, the aggro fiesta after the AoE fear is the #1 cause for wipes on that fight.

I would recommend Pala tank. It's great for the more casual content and casual player. It requires almost no effort to reliably keep threat on entire groups of mobs and as soon as you make a /cast [target=yourhealer] Blessing of Protection macro, you are skillcapped.
Additionally, it's pretty good in world pvp, as your damage is absurdly high and your survivability, atleast vs melee, is unparalleled.