Title says it..im just curious

Anyone?

Almost all. There are some opposing factions where you have to make a choice such as the Imperial Legion, and the Stormcloaks.

If a faction is joinable that means you can join it, yes.

I thought that would be obvious.

Technically, no. You can join the Legion or you can join the Stormcloaks.

However, because those faction's quests are under the same tab in the options "Civil War Quests", and you don't need to play both sides for the achievements, they are counted as one.

So yes, you can join all six factions on one character:
Bard's College
College of Winterhold
Companions
Thieves Guild
Dark Brotherhood
Imperial Legion/Stormcloaks

I don't recommend this, however. You should create six different characters specialised for each guild.

I believe at least two of them to be mutually exclusive.

You should not bump your threads after 3 minutes.

Just The Legion vs Imperials?

Or are there any other incompatible Factions? Quests in one which block off others in another? Anybody know?

Is that a serious question? I want you to sit down and think about what you wrote in your title.

Unless you meant as one character. In which case, you should've elaborated more.

Well, I dont support that. If someone wants to experince them with the same character, I'd say go ahead. For more pure RPG players that might seem like a crime, but after all, we all play the game the way we want. My first character is both devoted to the collage as well as the stormcloaks and very happy wíth those two choices.

You can join the Stormcloaks OR the Imperial Legion.
Other factions, such as the College,m Companions, DB, and Thieves Guild are all joinable no matter your standing with each other one.