The rack should set more than 2" or more above the bottom (to give room for liquids without braising the bottom of your roast), and the handles should be easy to grip while wearing oven mitts.
It also shouldn't melt at high temperatures!!!
An excellent site for reviews of kitchenware of this kind is:
Cooks Illustrated: Home
and has a FREE two week trial.
Cuisinart Chef's Classic Roasting Pan - $79.95
KitchenAid Gourmet Distinctions Roasting Pan - $59.95