Jeff,
Unfortunately, length would seem to be an integral facet of any "good challenge", but, it's the use of length that's critical.
I think you have to exclude one sub-set, the PGA Tour Pro and ask this question in the context of the amateur/club golfer.
The first holes at Friars Head, GCGC, NGLA and Sebonack all provide "good tests" despite their lack of length.
But, golf is cumulative test over 18 holes.
While each shot can be a test, and each hole can be a test, it's the cumulative test that's the critical one.
A given hole can provide a challenge to one golfer and be a pushover to another.
Ditto any given shot.
For the purpose of addressing your question, I think the word "balance" or "overall balance" has to come into play.
For me, the ultimate test is: Do I want to go straight to the first tee from the 18th green.
That question required a visceral answer, not a calculated answer.
I believe that opinions will vary on a given course because most golfers tend to evaluate a golf course solely in the context of their own game, rarely considering how the course interfaces with the games of others.
While length may not be a prerequisite, I think it has to be an element or facet at some point during the round.