I was on the Star earlier this year on one of the cursed (non)Antarctica itineraries. I've also been on the Sky a couple times in the past two years.
There were several things I disliked about the Star. The indoor areas were crowded, probably because it was a cold-weather route and there is no Observation Lounge. The buffet was sometimes packed to the point that there was no place to sit. Tendering was very, very slow. The internet was barely operational, though it may have since been fixed. The food and service seem to get more bad reviews than other ships. (I didn't have any major complaints in these areas, but I could tell there was some disorganization.) Overall, it was my least favorite of the four NCL ships I have experienced. I might be willing to give it another chance, depending on the price and itinerary -- ideally warm weather and no tender ports.
That being said, the Star does have several nice food and beverage options that the Sky does not. The complimentary (but often very busy) Asian restaurant, the whiskey bar, and the Brazilian steakhouse, for example. It also has a Spice H2O. I had four specialty dinners on the Star and all of them were perfect.
The Sky is a small ship so you tend to see the same passengers and crew a lot. The only thing I particularly dislike is that The Local is not 24/7 like it (or O'Sheehan's) is on other NCL ships. At breakfast/lunch it is used as overflow seating for the buffet.