Some points :
"In steels, strength is
directly correlated with hardness -- the harder the steel, the
stronger it is."
This is true for tensile strength, but for torsional strength (twisting) the behavior is very different, and adjusting the hardness up or down just 1-2 points can cause the strength to crash to ~25% of optimal. Alvin has talked about this a lot on rec.knives and hardens his knives to hit the maximum peak of the torsional strength graphs. The difference is *massive*, I have cut very "abusive" material with his very thin edged knives, thick plastics, bone, and even sod.
"Toughness is obviously important in
jobs such as chopping, but it's also important any time the blade hits
harder impurities in a material being cut (e.g., cardboard, which
often has embedded impurities)."
I thought this was true for awhile, but after doing a *lot* of cardboard cutting I think strength and wear resistance is more important here, I have lots of knives which won't take impact well, but will cut cardboard all day long. I think the chipping in cardboard is from failure from deformation + carbide aggregates and grain size issues at low edge angles.
"Some steels just seem to take a much sharper
edge than other steels, even if sharpened the exact same way.
Finer-grained steels just seem to get scary sharp much more easily
than coarse-grained steels, and this can definitely effect
performance."
Burr formation is a large factor here, as is how the knife is ground, and the latter is influenced by the steel, thus if a maker chooses the steel to the knife (not based on what is hot) and grinds accordingly, you should be able to sharpen it easily, thus an ATS-34 hollow ground knife could sharpen very well for a skinner, but horribly for a hard use tactical, because the steel properties well suit the first but not the latter. This is a complex attribute but essentially when considering ease of sharpening, the knife has to be considering just like when talking about "best steels" in general.
" D-2 is much tougher than
the premium stainless steels like ATS-34.."
In terms of impact tests it is similar, it also tends to shatter when broken, or fragment, as opposed to most tool steels which will break clean, many stainless steels are like this, when they break they leave lots of pieces and some can be projectiles, in general, the softer more ductile stainless steels like 440A break clean.
"However, M-2 rusts easily."
Compared to 1095 M2 is very rust resistant, and many tool steels like O1, L6, 5160 also rust very quickly, for the tool steels, M2 is one of the best at resisting corrosion, it doesn't compare well to the stainless steels, but they can also pit badly in extended salt water soaks, something that the tool steels don't - so there is a difference between resisting short term surface corrosion and long term pitting damage.
[S30V]
"This steel gives A-2-class toughness and almost-S90V
class wear resistance, at reasonable hardness (~59-60 Rc). "
S30V's toughness is only A2 level along the grain which is never loaded in impact, along the grain which is, it is similar to the other steels and it doesn't respond well to impact. I have broken several high end S30V blades by impact and it is easy, a light pop with a hammer. The wear resistance is also a lot lower than S90V, only about 30%. There is also some promotion of CPM-154CM now to replace S30V or at least be an alternative due to its higher machinability, plus that section of the FAQ is a little dated as S30V has been out for a few years now and thus its perforamnce is very well known. The hardness range used is also wider, ~55-63 HRC. Reeve hits the lower end on his fixed blades and Wilson the higher.
"Mission knives uses titanium."
They use Beta-Ti which is much stronger than the low end Titanium knives, it is very resistant to impact, you can actually cold work it, and extremely flexible, it won't break by bending, you actually have to *rip* the blade apart.
Great reference as with all the FAQ's.
-Cliff
I haven't made distinctions in the variou stypes of strength, just generally looking at when an edge seems to blunt or indent. 
. We have been chatting at length about the possibility of a new, top notch Hawkbill blade on the Spyderco.com Forum. I just recently suggested the use of S30V as an ideal blade steel mainly based on the excellent luck that I have had with my Spyderco Dodo with S30V.