(I know I'm setting myself up for at least one shot at me)
My #1 biggest pet peeve is C# naming conventions!!
Field Variables
Man does it drive me bonkers to find field variables with a prefix like _MyVariable or even worse m_MyVariable. Arrrgh, it hurts just to even type it! What does the underscore do for you? Tell you that it's a field variable? Why not have ÞMyVariable or or zæMyVariable? Those are just as arbitrary as an underscore or m underscore. If you can't determine the variable is a field variable from the code you're in, I'd say that your method or class has become too bloated and could use some refactoring. Personally, I stick to field variables using camelCase and if it needs it, the property accessor with PascalCase. So myVariable and MyVariable. No other crazy characters needed!
Reference Types
If it isn't a value type, then it's a reference type and eventually down the inheritance tree its great grandpappy is Object. Why oh why do I run across stuff like Customer objCustomer = new Customer(); I look at this and read "Object Customer is a Customer Object". Come on people, this isn't an untyped scripting language!!
So, what gets your blood boiling out there in coding land?