Too soft, and your sword gets cut into. Too hard and the sword is too brittle. Poor heat treating can totally ruin a sword. It's amazing how some businesses or websites boast about the quality of the steel but speak very little about how a sword was heat treated. If you don't heat treat it right, initially, it might not survive the final water quench and end up crack.
Some cracks are very obvoius, and some are very fine.
The fine ones can grow larger over time in some cases. Even if some have a good concept of the first two points, some sword makers don't have any concept with how a sword should feel. The best thing to do is to discretely ask the sword maker his philosophy on approaching a blade design. Dean Piesner, a sword maker in St. Jacobs, Ontario, Canada, relates that he first determines with the customer what the intended use of the blade - and its intended target is. A sword made to cut armor will differ in design than a sword designed to just cut through flesh.