What Exactly Are Reeds??

See Eric Gorr.

Reeds do two things. They allow the air/fuel charge into the engine, and KEEP it there on the piston downstroke (when the crankcase is being pressurized to move the air/fuel charge thru the transfer ports to the combustion chamber).

All of the things you mention (speed, rpm and powerband) can be affected by the reeds. The angle of the cage the reeds sit on, the tension of the reeds (how stiff they are), the number of reeds (and therefore the size), whether they are a two-stage variety or not......all of these characteristics have an effect.

re: 'doesn't seem much to them.'

That would be an observational error.

Huge numbers of man-hours on the technical side of things (research) and the butt side of things (test rides) have been spent in finding out how to make these simple things work. Take a look at the oem reed cage and the composition of the oem reeds and how they are attached to the oem reed cage. Compare that assembly to something like a DeltaForceII cage. The angle of cage is different. The reeds are of a different construct material. There are more reeds. The attachment mechanism is different...and adjustable, even.

Not much to 'em?

There's plenty to 'em.

Take just one little item for example.

Think of how two reed types would compare, one being somewhat more stiff than the other.

A less tense reed will open easier cuz it bends easier, right? So, it will 'open' with less pressure applied meaning it will open earlier than the other 'stiffer' reed.

What will you get bike-wise? Better bottom-end response.

BUT that same low tension reed will have a problem when it comes to high-rpm operation (compared to the higher tension reed). Think of the 'blast' of air/fuel that is being required by an engine spinning @ 10K+ RPM and how fast those pulses are occurring. The higher tension reed will do a better job of returning to seat on the cage than the lower tension reed will. If the reeds don't seat well on the piston downstroke, where's your air/fuel mix going to go?

Not where it should (above the piston thru the transfer ports)!

What is the bike response going to be with a higher tension reed? Better top-end response, but degraded bottom-end response.

That's just a glossed over quick look at one simple reed characteristic and how it works.