In the "Describing the Effects of Damage" sidebar of the Players Handbook, there is verbiage that PCs should know when the creature is half damaged. I find this reasonable. There should be obvious clues that the players are making progress.
The concept that hints should be given for resistance or vulnerability would also seem to imply that 25% damage is something that should be known, or 10% damaged, or 2%, etc. In other words, where does the concept of "less damage than expected" come in? Is 10 points of damage on a 30 HP creature any more or less the expectation than on a 400 HP creature?