I have never been a step-parent myself, but have had 3 different step-parents over time! I think the main challenge, if the child still has strong relationships to their pre-divorce parents, is to find a way to have a new parent-like relationship without threatening or trying to replace the existing bonds. I think this is particularly hard for step-parents who do not already have children of their own.
I have tremendous respect for how difficult it is to become a step-parent. I think it's somewhat odd and unfortunate that there is no real positive model for being a step-parent - even now, step-parents are often made out to be evil in popular culture. It's a challenge for a step-parent to develop an approach to step-parenting that finds the right line between being a good parent and not over-stepping.