Why does reentry from space tend to result in such great heat? Let's pretend for a moment that the atmosphere had  sea-level density, pressure, and temperature all the way up to, say 500km high, and then would abruptly end in a complete vaccum. In such a situation you could have a glider orbiting at 501km high which could do a very graceful, low temperature reentry by simply slowing down the angular velocity and then using aerodynamic lift at, say, a 1:30 gliding ratio to drastically reduce the rate in which potential energy gets released. Or even have a completely vertical entry using a simple parachute..
With our real atmosphere though it seems that all reentries are at very steep angles and at very high radial velocities. Why? Is this a direct consequence of the low density at high altitudes, which necessitates a far higher velocity to reach the same lift/drag, and that high velocity contributes to increasing the temperature far more than the low density contributes to decreasing the temperature?
 A: 
by simply slowing down the angular velocity

As simple as this sounds, it is a huge problem in practice.  The orbital speed is roughly 17,000 mph.  To reduce this to subsonic speed all the while maintaining altitude would require a rather large reentry "stage" similar in capability, I would imagine, to the third stage of the Saturn V.
Keep in mind that the space shuttle reentry engine burn lasted roughly 3 minutes and reduced the orbital speed by less than 200 mph.
We want to use the atmosphere to convert our kinetic energy to heat so that large "descent" engines and quantities of propellant are not required (as they are to land on an airless moon or planet).

With our real atmosphere though it seems that all reentries are at
  very steep angles and at very high radial velocities.

I don't think that is the case for controlled reentries.  If I'm not terribly mistaken, reentry angles are quite shallow, e.g., 6 to 7 degrees, i.e., the tangential speed is far higher than the radial speed.  
A: When a gas is compressed it gets hotter.
You can easily see that with a simple bicycle pump.
Now if a gas hits a surface at many kilometers per second, it gets very hot.
Hot enough to glow.
Check out Stagnation Temperature.
