It's hard to really identify absolute specs that determine what makes an excellent sword and what does not. However, a basic guideline to follow:
Workmanship. When you look at the fittings, do they look high quality or like cheap casts or forgework? Detail should be clear, shapes should be even, and it should look very well made. Look at the handle...its the ito wrapped tightly, does it feel comfortable, are the diamonds even and proper? The rayskin underneath should have nice *big* nodules. The fit between the tsuka, tsuba, seppa should all be even and line up well. The saya should fit well...gentle and snug, hugging the habaki enough to keep it in and free from rattling. The blade should have crisp lines, in looking down it you should not see noticeable wobbling or dips. The surface will be even enough that you can probably read off the reflection in the blade. There will be no secondary bevel edge, and it should be sharp. There should be no readily visible scratches, and the yokote (if present) should be well-defined and placed with the proper corners (mitsukado, ko-mitsukado). If the sword has a horimono (carving), it should be tasteful and well executed in 3 dimensions, not just a few carved characters or a vague outline of a dragon or what-not (things that are often signs of a poor quality mass-produced Chinese blade)
The sword will be substantial, yet light enough to use quickly (this is variable depending on what you need in a sword specifically). Balance closer to the handle does not always mean better, neither does light weight.
The hamon will be soft luminous white, while the yakiba will be a lighter gray and the ji a bit darker. The shinogi-ji and mune are typically burnished and therefore have closer to a "mirror" finish. If there is hada, it should not be loose, and there should be few open flaws (slag inclusions, etc). Fine grain does not necessarily mean "better" but it can be desirable. When looking at the hamon, you will either see it as nioi, which is like mist (very fine) or as nie, which is closer to crystal sugar-looking (more coarse). Highly coveted blades tend to have both of these crystalline structures present. There are a number of types of activities in the steel such as ashi (legs extending down from the hamon) or utsuri (a "ghost" hamon higher on the blade) that are also well-regarded in sword circles.
Of course, none of this *necessarily* makes an awesome sword...it could in fact be a blade that will chip if you look at it wrong. However, if you can assume that the blade is reasonably well-made...the things above are all elements that make an excellent "complete" sword.