Well I've personally never done the actual recruitment process myself but a few of my colleagues have been asked by HR to help out with the application screening process.
Big 4 accounting firms look for people who have strong academics, usually credit (65%+) average (however, that isn't to say they've never hired people with less than 65% - it just depends on the entirety of your CV), prior work experience during their tertiary studies (doesn't have to be in accounting roles, but at least some indication that they are not just book worms, as they want to hire someone who can handle working full-time and studying their CA) and extra-curricular activities such as executive positions in Tertiary clubs or volunteering work. However, I must state that for roles especially in Corporate Finance, you would need to have at least 75%-80%+ average to have a chance (what you would expect from IBs because most of the candidates that apply for Corporate Finance will most likely apply there). For those that intend to work in Corporate Finance in Big 4 accounting firms next year, there are very limited positions in each firm e.g. 3-6 (max) - usually taken up by interns. I know because I work in Corporate Finance at one of the Big 4s and I've heard rumours that the intake is limited.
Usually for Big 4 accounting firms there is a screening program (like those of many Big 4 banks and IBs), which pick up specific words that they feel is key to a candidate. There is no way in hell a HR person can sit through 3000-4000 applications within 1 week and give you a call back to say you've got an interview.
In the interview, there is a "100-point system" (or similar) that is used by the interviewer to assess whether or not you are the correct fit to the role and to the firm. This usually means that the firm's "culture and key values" are a set of criteria, which you're assessed against. At the end of the day, the interviewer is looking for someone that they can work with.
In next years recruitment drive (graduates for 2010), there will DEFINITELY be far less positions available to graduates because of the credit-cruch hitting corporate earnings, hence Big 4 accounting budgets and profits. In addition, IBs are not hiring as many due to their international counterparts culling staff and a flow-on effect of "potential" IB candidates flooding to Big 4 accounting roles. If I were a penultimate year student wanting to work in Big 4 accounting, I would be gunning for an internship position at a Big 4 accounting firm to secure a place as a graduate in 2010. I personally think the markets will be depressed for 1-2 years.