I have only just started Skyrim so my background in TES lore is minimal, what is an Imperial and why would I want to play one?

Imperial is one of the 10 playable races from Morrowind to Skyrim. I am most certain you have seen an Imperial in Skyrim

OK, why is being able to play one an advantage? What aspect of them is causing the uproar?

It's not being able to play one that is causing the uproar, it's the fact that you have to pay extra to play as one.

*10

Playing as an Orc and Imperial was new when Morrowind came out (You couldn't play as one in Arena and Daggerfall), ever since you can play 10 different races in Oblivion and Skyrim. When TESO was announced, the Imperial race was no where to be found and most thought ZOS just put them aside as a non-playable race.

As QuickSave stated, the uproar is that the Imperial finally gets some light, but you have to pay extra for it.

Aside from the bonuses you get alongside the ability to play as one (e.g. creating Imperial style armour, exclusive mounts and XP bonuses), you can choose whatever faction you want to play in. It's causing an uproar because a game that will already have a box price and subscription is trying to place content (in this case, one of the most distinguished races in Tamrielic Lore) behind a paywall in the form of the Collectors Edition.

We always knew the Imperials would be in TESO, but we were told we couldn't play as them as we're fighting against the Empire. They're essentially the bad guys.

the daggerfall and arena thing is because if Im correct the imperials weren't a thing when arena and Daggerfall came out because the lore was just started. and orcs were simply monsters in the world.

The issue I have with your statement is that races, and classes, are "features". They are something you get to "be" in the game. And "content" is something that you get to "do" in the game. And while this difference may seem subtle to you. They are in fact distinct. So when ZOS said they would not charge for "content" they meant they wouldn't charge you for access to an Adventure Zone. Or access to areas of the map. They are examples of "content".