Stayed an ensign for 7 years and never got promoted.
Got an official reprimand on his record for fucking an alien and getting a luminescent STD
I do think he mentions this to the captain at some point (normally ensigns are promoted between 3-5 years of service) Meanwhile Harry had been serving non-stop for 7 years.
I mean he did keep playing his clarinet, but surely his parents would not approve one bit and seppiku must be demanded!
Even Tuvok got promoted from Lieutenant to Lieutenant Commander, although that rank is appropriate for the third-in-command of a starship; Data on TNG was a Lt. Cmdr, along with Geordi, Deanna until mid season 7, and Dr. Crusher. Of course the Enterprise-D has a crew that's almost 7 times as large as a typical Intrepid-class starship.
Kim definitely has the experience to warrant a promotion not just in years, but also in other ways; in season 7 "Nightingale", he takes command of an alien vessel as acting captain, and he serves as the officer in command on the bridge in several other circumstances as well (like when Janeway doesn't feel like it, Chakotay is off trying to woo her, and Tuvok is meditating or playing Kal-Toh or something).
I mean, he was also Tom's sidekick in his Captain Proton adventures, and also managed to fall in love with a Borg, the wrong Delaney sister, a hologram, and the deceased returned to life. It's a wonder he didn't Seppuku himself at some point in the journey.
Poor, poor Kim. Garrett Wang deserves better.