Keeping the blade in the closed position is one purpose of the detent, and in that regard I agree with most others here that if it can shake open or come open when I do not intentionally open it, then it is too weak. However, the other purpose of a detent is as part of the opening mechanism, at least on most folders, and it's in this regard that weak detent strength is a tricky subject.
From the perspective of knife manufacturers/makers, I've heard that this is a problem which doesn't really have a solution. Each person has their own preference, and a hard detent for one person is a normal one for another. Take my brother for instance. He's not into knives, but I do show him new acquisitions on occasion. He can somehow almost always get the blade to not fully deploy, whether it's a flipper, front-flipper, thumbstud, aperture, etc, and thus says a number of my knives have a weak detent even though some of them are widely acknowledged by us knife enthusiasts as finger breakers.