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Now, there's obviously one use for this section insofar as seeing which tire is breaking away first, but attempting to understand it better, I've come to 2 questions:
a) What exactly is this thing telling me? Not just the percentage numbers, but the circle sizes, and those other things that seem to point in the direction of the force, but also change in size
b) Is there actually anything useful one can do with it other than deciding where the diff is on accel, and seeing that the car is under/oversteering or locking up the front or back first?

I'm looking at it, and if I assume that the circle size has something to do with the load on that corner (eg. outside front=big circle, 110%, inside front=little circle, 150%), it almost makes me want to put roll stiffness in to try and even things out. But it also seems like that's just going to make the front end plow harder.

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The bigger the green circle is, the more traction is available at that tire. This is, of course, directly proportional to the amount of vertical load on the tire. If the circle turns red, you are obviously exceeding the amount of traction available.

There are also two lines that come out, and change direction and length. It is unclear to me exactly what this are, but may be the direction the tire is pointing, traveling, or may represent slip-angle somehow. I found some very old threads that discussed this, none of which came to definitive answers. I would like to know more about this myself.

carbsmith:

b) Is there actually anything useful one can do with it other than deciding where the diff is on accel,

The best diagnostic for your differential is your individual wheel speeds. If they are the same on one axle, then that differential is locked.

 and seeing that the car is under/oversteering or locking up the front or back first?

I pretty much only use that menu to analyze load-transfer and to set my brake bias. It can also be used to tune the driver, i.e. show where you should lift.

 it almost makes me want to put roll stiffness in to try and even things out. But it also seems like that's just going to make the front end plow harder.

Adding roll stiffness will make things less even. Roll bars do two things, one of them bad and one of them good:

1. They increase lateral load transfer. This is always bad.

2. They help maintain proper camber angles. This is good. But the telemetry you should look at to guide your decision on this is tire-temperature profiles. Some people also slow-mo the dynamic camber telemetry (I do not).

3. I know I said two things, but they also affect your oversteer/understeer.

So run the minimum ARB you need to maintain proper camber angles and you should be good. This will vary by track.

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Click image to see the car's sale thread.
GT: EXOR x Bart x
http://www.exodusracing.net/ - Unassisted Racing at its Finest

Yeah, logically I decided that was probably off. I was probably thinking more in terms of body movement on the sprung mass than actual load transfered to the unsprung mass. Obviously, if the thing just keels over on the outside bump stop, you're suddenly shoving a ton of load down on that tire, however, but I don't think that's a significant worry in FM2. I almost sort of wish it was showing contact patches. That'd be more useful than the numerical camber angle, anyways. It's not desperately difficult to figure out which tire is screaming and/or pluming smoke after all (even passively, tire temp will tell that tale), although the lack of visible body movement makes some of the weight transfer conclusions harder than they should be.

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The lines within the circles show some sort of lateral force. If the yellow part of the bar crosses the green ring, you have lost traction. The bar goes up when you're going forwards, back when you're braking/reversing and left and right accordingly.  *shrug*

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I'll send a message asking for explanation and see if T10 is kind enough to answer.

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Click image to see the car's sale thread.
GT: EXOR x Bart x
http://www.exodusracing.net/ - Unassisted Racing at its Finest

Gotta love it that they didn't fully document their features :)

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when i am tuning a car, i usually look at the friction thing to tell if i have the springs set up where i want them, when i go into a tight turn, i want the rear to break traction just before the front, also, you can tell if you get a wheel coming off the gound when the circle becomes a dot, and the percentage drops to zero.

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first release
GT: MasterSmite0wpi

Besides the other uses - I use the friction telemetry to tell me about the bump and rebound settings.

A tire that loses grip as the wheel goes light over a bump has too much rebound. Reduce it a little and see if you can get the tire to maintain grip.

Same goes for braking. Losing traction at the rear when under braking can be a sign of too much rebound. Try to reduce the rebound and see if the tire maintains grip

A tire that loses grip as it hits a bump or a front tire that loses grip as the brake is smoothly applied (but the shock is still green) has too much bump. Same goes for acceleration. A smoothly applied throttle, with a green shock, should not cause the rear tire to lose grip. If it does the rear bump setting may be too high.

The size of the circle relates to the overall amount of grip the tire makes. To increase the diameter reduce the spring rate but only to the point that the car becomes unstable when the suspension bottoms out.

Forza only allows tuning front and rear and not each wheel independently so both rears must be tuned the same and both fronts the same so find the optimum setting for the specific track. Lots of long, fast, right hand bends before long straights mean you should focus your attention on the left side of the car and vice versa. Remember: you'll make up more time coming quickly out of a corner that's before a long straight than you will going quickly through a corner that leads to another set of corners.