Elisabeth Hendrickson wrote about a case of complacency and lack of imagination in a tester she interviewed.
I agree with the gist of her argument, that good testers have a "knack" for considering myriad ways to approach any particular situation.
But I would also like to point out that just as a "tester who can’t think beyond the most mundane tests" is dangerous, so a tester who assigns equal importance to every possible test is just as dangerous, if not more so.
The set of all possible tests is always immense, and the amount of time to do testing is always limited. I like to encourage a knack for choosing the most critical tests. Sometimes this knack looks like intuition or "error guessing", but in those cases it is almost always, as Brian Marick put it in his review of Kaner's "Testing Computer Software", "past experience and an understanding of the frailties of human developers" applied well.
Intuition and error guessing take you so far, but there are other techniques to limit the number of tests to those most likely to find errors. The diagram here represents work I did some time ago. I was given a comprehensive set of tests for a particular set of functions. Running every test took a long time. In order to arrive at the smallest possible set of tests that exercised every aspect of the set of functions, I arranged a mind map such that each test whose test criteria overlapped the test criteria of another test was put farthest away from the center. Each test whose test criteria included multiple sets of test criteria was put closer to the center. In this way, I arrived at the smallest possible set of tests that covers at least some aspect of all of the set of functions under test.
It is not a comprehensive set of tests. Rather, it is the set of tests, which, given the smallest amount of testing time, has the greatest chance of uncovering an error in the system.
There are certainly echoes here of model-based testing, although I think this approach plays a little fast and loose with the rules there.
Regardless of whether the tester chooses a minimum number of tests through intuition or through analysis, it's a good skill to have.
And if you do encounter a tester who creates a smaller number of tests than you think is appropriate, you might want to check his reasoning to be certain that that tester is really making a mistake.