I'm looking at a used powermeter crank. It is a "Hollowtech" bottom bracket type. My knowledge of bottom brackets begins with cotters and ends with square taper. Will this crank fit in a traditional 68mm English thread bottom bracket shell?

In fact, just to save me from asking again - what are the modern, new-fangled (that is, post C&V) cranks and bottom brackets that will NOT fit in a traditional 68 mm English thread bottom bracket shell?

Press fit bearings.. shell is machined then, groove for snap ring..

as fiestbob says, oversized shell bottom brackets (I think they're mostly press-fit, but I haven't worked on them) are a different standard. But most stuff is still available for 68mm English threading.
so yes, a hollowtech thread-in will work for standard English-threaded 68mm bottom bracket. it'll look funny to some people but if your frame is good enough to warrant top new-fangled stuff, don't worry about it.

I put a hollowtech on an old olmo and love it. I wanted newer stuff on one of my classics for when I wanna feel racey with all the gears.

[MENTION=200660]triathloner[/MENTION], I'm curious how much of a difference in stiffness you notice with the new crankset on the old frame. My sense is that most of the BB flex on older bikes comes from the frame, not from the crank and bb, but some of it is still from the crank and bb.
Sounds like the biggest difference you notice is more gears.

I would say that it is the same either way, I didn't notice it changing, so if ther is a change it is probably small.

Ok, so I just buy the appropriate threaded cups for the Hollowtech. Thank you!

yup. good luck.