Bill, here's the corresponding rating in Charles Wells' guidebook for those trails, listed as "Easy (E)", "Moderate (M)", and "Difficult (D)". If any part of a trail gets the higher rating, the whole trail is listed as such, so many tougher ones are doable with some assistance for a small portion.
Hell's Revenge - D - Steep inclines and ledges
Fins and Things - D - Steep inclines, sandy sections, some rocks
Cliff Hanger - D - Many tall obstacles; not suitable for stock rigs
Poison Spider Mesa - D - Classic hardcore trail, many tough obstacles
Golden Spike - D - Same as PSM above
Behind the Rocks -D- Some tough, intimidating obstacles; lockers recommended
Gold Bar Rim -D- Starts easy, gets tougher as it goes, stock can probably make it, route is hard to follow at times
Cottonwood Canyon - Not ran in book
Onion Creek -E- Nice scenic drive through a narrow canyon, with multiple creek crossings; leads to other trails, such as Rose Garden Hill and Thompson Canyon
White Rim -M- A very looong trail, mostly easy, with a few moderate obstacles, narrow at times with long drops: the length probably rules this out (it's about 90 miles long!)
Elephant Hill -D- Steep, narrow, rocky climbs and descents; recommended not for beginners; long drive to get to this one--will burn a lot of time
Beef Basin - Not ran in book
Kane Creek -M to D- Narrow at times, steep drops, some ledges, many water crossings; should be fine for stockers as long as it hasn't rained recently
One definite thing to consider as well is ease of route-finding--staying on a trail on slickrock can be maddening, with few markers on some trails. The scenic drives seem to be okay, though, as they usually follow "roads". I would highly recommend C. Wells book by Funtreks, available through many sites--this way those interested can read firsthand descriptions of what they're in for, and we can all approach this from a slightly more educated perspective.