The heavier the better, as long as you can lift it. The first feature that everyone mentioned was heft. Not heavy for heavy's sake, mind you, since in many cases you'll be lifting the pan along with a multi-pound roasted something. You want good heft for two reasons: you don't want a hot and heavily loaded pan to warp, twist, or flex; and you do want even heat distribution so your precious drippings don't burn during roasting and saucemaking.
These criteria point to a couple of metal choices: heavy-weight stainless or copper. Enamel-coated cast iron is hefty, but too much so; the pan would just weigh too much to be practical. Regular aluminum has the potential to react with acidic ingredients, and it seems to warp more, even in heavy form. Anodized aluminum can be good, but the dark interior wouldn't be my first choice, as I discuss below.
Nonstick is not an advantage. My preference in cookware is not nonstick (except my omelet pan). This is partly out of habit and partly out of performance concerns: if you want to deglaze (and for most roasts, you will), you need a surface that encourages the juices to adhere and develop deep flavors. Nonstick does precisely the opposite.
And most nonstick is pretty dark (as is anodized aluminum). A light surface helps you to judge the character of any juices (are they getting too dark? is the chicken juice running clear yet?). I suppose if you're using the pan to bake a mega batch of potato-and-cheese gratin, you may have a few more scrubbing issues, but none of the cooks I talked to was a nonstick booster, for the same reasons I gave.
The biggest is not the best. I want the pan big enough to accommodate the largest item I'm going to roast, and by accommodate I mean allow the food (most likely a 14-pound turkey) to fit in the pan without touching the sides and with enough space for air to circulate and brown the undersides. I also want the pan large enough to work as a water bath for eight ramekins. But I don't want it so big that when I roast a chicken or a pork loin there's so much exposed pan surface that the juices will burn. (If I'm roasting something very small, I'll just use a heavy skillet.) I fill any empty space in a larger pan by tossing in a few vegetables and hardy herbs. A more moderate size will also make the pan more appropriate to use for a shepherd's pie or a cobbler. I'd go with something about 16 x 13 inches. Before you decide on size, measure your oven: some wall ovens are surprisingly small.
Side height is critical, too: too low and you risk sloshing your hot liquid when braising or using the pan as a water bath; too high and the hot air can't get to the lower areas of your food during roasting. Three-inch sides seem a good compromise.
Think about shape. I'd choose a rectangular shape with rounded corners (so a whisk can reach in easily). Oval roasters are pretty and they work well with oval roasts, but they're usually not capacious enough for roasting two chickens side by side or six Cornish hens, or for fitting in a whole batch of crème caramel.
The last big design feature you need to make a choice on is the handles. Given that in many cases I'll be lifting this roasting pan, loaded with a multi-pound cargo, out of a 500°F oven, my choice would be for a pan with thick, riveted, fixed handles rather than the sliding "bale" style. The fixed handles take more depth for storage, however. I'd also look for a model with a bit of flare to the rim so that I could crimp foil over the top when braising or cooking casseroles.
