For me, English; I did English Language and English Literature at A-level (having only studied English GCSE) and the gap was enormous.
English Lang: way too technical from your similes and metaphors and writing to persuade and inform, etc. Nothing like that. You're getting told about theories that you thought was only studied in Psychology and Sociology and etc. The way you have to analyse and omg, I only thought there was one adjective. But nooo! It turns out there's a ton of different forms of adjectives. The amount of grammar and syntax you have to remember and analyse was almost fatal for me.
English Lit: the reading got way heavier. Esp the texts, I thought were so easy at GCSE, compared to A-level. AT GCSE, I thought making chapter summaries for Of Mice and Men was heartbreakingly tough, let alone ripping to shreds paragraphs after paragraphs of Birdsong! The amount of detail you go into, is so enthralling but very hard. I also learned that it's one of those subjects that if you don't continuously practise at, you will fail. You can follow the same essay plan guide at GCSE at write a dozen essays on different texts and get the same consistent grade. However, at A-level there's a thousands formulaic ways of writing different essays for different genres that will always be a different grade, depending on how much you put into it. The terminology gets a lot heavier and harder, especially for the poetry side of A-level.... Why am I going to uni to study English Lit again? :P