I don't actually remember which AS topics I liked/disliked. Just finished my first year of uni, so it was a while ago. But generally:
Biology:
AS is initially quite a big shock because of the large number of new words and new topics. The problem with the GCSE --> AS transition is that you tend to learn completely different things in AS, so everything is new as opposed to a continuation of what you already know. The content itself isn't particularly difficult, there's just quite a lot of it. As long as you review your notes every now and then, it shouldn't be too difficult to cope when exams come around (the biology exam is essential one big memory test). A2 isn't that different to AS (more new words, more new topics), but the topics are a little more difficult in the second year.
Chemistry:
Chemistry, in my opinion, is actually a smoother transition than biology. The first few weeks covered a little new content, but was mostly a review of what we'd learnt in GCSE. It's different to biology in the sense that you learn concepts and then apply them throughout AS and A2, rather than learning many things that seem to have little relationship to one another. Basically, if you understand what you're being taught, it should be fine. If you don't, you might have a problem. A2 chemistry is a bit of a step up, but the same concepts. As long as you have a firm grasp on AS chemistry, A2 isn't too bad, because once again you're just taking what you already know and applying it elsewhere. I think the general consensus is that biology is easier than chemistry, but personally I found it the other way around.
Geography:
I don't really have much to say about geography. It's much the same as biology, in the sense that you just remember it all and write it down in the exam. I didn't really feel the jump between GCSE and A level. In fact, two of the AS geography topics were actually the same as two of the ones I did for GCSE. Personally, I didn't particularly enjoy the subject, so I dropped it after AS, but I think that was my problem rather than the topic itself.
The workload is a big step up from GCSE, but it's manageable (especially compared to university). As long as you start your homework early and work efficiently, you shouldn't have a problem keeping up. Most who struggle with the amount of work do so because they leave things until the night before they're due and then realise they don't have enough time.
GCSE results: 9A*, 1A, 2B
AS: 4A
A2: 3A*
General advice for revising would be: start early, try to understand what you're learning rather than just trying to memorise it, use the online specification as a checklist for what you need to know, familiarise yourself with the past papers and mark schemes. Above all, ensure you know what works for you. Some people can just sit and read a textbook and everything goes in; others need to condense things, make notes, draw diagrams etc.