When does it start get really intense (like 2 or 3 weeks into it) or is it generally laid back (is it as "stressful" as yr 12 was?)
And does anyone know if marketing, management, psych and academic writing is hard or not?
I can only speak for my own faculty, but for Arts, things really start to pick up in Weeks 8-12. That's when you're going to have several major 2000+ word essays due, and if you're really unlucky with your unit selection they may all be due within days of each other. As long as you plan how you're going to tackle this well in advance, it shouldn't be a problem.
Honestly, uni is about as intense and stressful as you want it to be. Nobody's going to hold your hand and tell you when your essays are due and demand that you turn up to lectures. Personally I never found uni as difficult or stressful as VCE, but then again I'm just a straight Arts student.

I took Academic Writing back in 2009 and I found it very dry but incredibly easy to score good marks in. So if you're looking for a bludge subject, you picked the right one, haha. I looked at the reading list for Academic Writing this year though and it seems they've revamped the unit significantly; it actually looks easier than in 2009.

So I doubt you'll have a problem there.