Test::Unit is simple, comes baked in, and because it's not some faux-English DSL your brain isn't constantly fighting with itself making stupid and incorrect assumptions about syntax because 'hey, it looks like English'.
The worst testing-related decision I ever made was using Michael Hardt's Rails Tutorial (which has RSpec baked in) to learn Rails in the first place. Great tutorial otherwise, but it set my testing back immensely - it pretty much meant I did no testing at all. When I finally bit the bullet and decided to learn how to use Test::Unit, I was astounded at how easy it actually was.
Most Rails resources online assume familiarity with so many not-out-of-the-box tools it's a bloody miracle anyone can learn Rails at all. It's a real problem.