Class 2 in the High Sierra is difficult cross-country travel, usually with talus hopping and the occasional use of hands for balance. The talus can be unstable, and the danger is that "hikers may stumble among these blocks". He goes on, "It is also possible for a boulder to dislodge and roll over a hiker."
Class 3 is where you need hands and feet to hang onto the rock. Steep or large talus can be class 3. "Novices may feel uncomfortable, but the holds are large and easy to locate."
Class 4: "steep rock with smaller holds and a lot of exposure... A fall will probably be fatal."
So exposure seems to be a factor when going from class 3 to 4. But not a factor in class 2. And it isn't clear to me from what he wrote that a route can be class 3 just because it has steep or large talus (and loose!), but if it can, then Wallace Col should be class 3. You might be able to do it without any real climbing. But then class 2 can mean a lot of different things, can't it, and you just have to know that. Echo Col is a clear class 3, as the novice who accompanied me last summer will testify to. I think Lamarck Col is class 1-2, much easier, and nothing scary. Black Giant should be rated class 2. Secor was joking when he rated it class 1.
@cinematic: great story!
-Eric