How 'space ready' was space shuttle prototype Enterprise?

Question

I have long believed that Enterprise was just an empty craft, with only the instruments to operate in shorts flights in the atmosphere.
Basically a large glider with odd wings.
However, I have been seeing Enterprise referred to as not space-rated or space-ready, and sometimes as an 'orbiter'. (For example Space Shuttle Orbiter Approach and Landing Test: Final Evaluation Report that is linked in @OrganicMarble's answer calls it "Orbiter 101" a few times)
This makes it seem like Enterprise was actually a complete Space Shuttle, but missing some testing or without a few required space elements.
How much of a prototype actually was Enterprise?
It apparently did not have engines, but the other shuttles also needed to refurbish the engines after each flight.

Organic Marble · Accepted Answer

Your belief was pretty correct.
Major systems missing on Enterprise:

orbital manuvering system
reaction control system
main propulsion system
space radiators
airlock
star trackers
thermal protection system (tiles)
cryogenic tankage for fuel cell reactants
payload bay payload mounting hardware
galley
aft and overhead windows
Ku-band antenna

Major systems different on Enterprise:

hydraulics
electrical
thermal control
cockpit controls and displays
structure
communications

Enterprise-only systems

nose-mounted air data probe
high pressure tankage for fuel cell reactants
flight test instrumentation
umbilical to 747 mothership
tracking beacons
ram air scoop

She was intended only for testing the final stages of aeroflight1  so had only systems required for that installed.
1  The original plan was to convert her into a spaceworthy orbiter by adding the missing systems but it ended  up being too expensive, largely because of structural design changes to the wings, mid-fuselage, and thrust structure. Challenger was built out of the structural test article instead.

References

Wikipedia (partial list)
Space Shuttle, Jenkins, 1992 edition, p. 138 and 147
Space Shuttle Orbiter Approach and Landing Test: Final Evaluation Report

How 'space ready' was space shuttle prototype Enterprise?

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