My y12 dd has always been a fairly hard worker, but she's working much more this year than she had to before.
The differences I can see with the subjects she's doing to AS are -
French: more tenses and grammar matters more, but also the nature of what they're doing is different. They'll be reading a set text shortly, though don't know what it will be yet, but instead of writing little passages about Le Sante or whatever, she's writing essays and has found that the biggest difference is that she has to argue in French as she would in English or History. So she's getting marked on how well she presents the case, is needing to say things in French which connect, develop or advance her argument - and that seems to matter as much as knowing the vocab. French is probably the subject that's caused her the most angst so far.
Maths: she has found this, I think, the easiest transition in some ways, partly perhaps because they did the Further Maths GCSE last year. However she's been doing Mechanics for the last several weeks, and doesn't like that so much. Style of learning and delivery and content seems least changed.
English - from doing both Eng Lit and Lang, it's been very different doing only Lit (which is what she chose). Whilst I wouldn't say it's like university work, there's clearly an attempt to make it so, with examination of selected critical passages on the texts they're reading (which they've had to do much more in their own time, which also adds to the work load). Quality of the writing matters more, it seems - she was perplexed recently to lose marks for writing although her argument was ok, because she kept saying 'this' without saying what 'this' was, for example.
History - as far as I can see, same sort of approach but longer pieces of writing and lots of timed essays. DD likes timed essays, but many do not.