One of the big changes I remember hearing about, is if your GPU is so old it does not support Shader2.0 functions. HTC ran a query and a very small set of systems fall into that category. Other aspects of the new engine, such as terrain mesh detail and the like, might cause some rigs to puke due to lack of resources.
It's possible you may need to upgrade some aspects of your rig, but I'd hold off until release, or the new minimum specs are published.