Being a contractor in the UK for a couple of years now and from what I understand (not an expert, thats why I hire them) are the following:
1. Length of Contract can be an issue. You need to be careful if a contract is over a year.
2. If you are in a management position. i.e. responsible for permanent personnel; doing reviews, holidays etc..

Im sure that there are other rules, but Im not clear of them, also from what I understand and have read even after a couple of years with IR35 in place everyone is still confused on this, from HR, Lawyers, Accountants and even the HMRC.