Grooming, snow quality, trails, scenery, staff, lodge.
For a day skier, none.
For Midwest skiers and snowboarders who deeply care about the sport itself and aren't terribly concerned with the fluff of glitzy resorts (fancy restaurants/hotels, on-site masseuses, etc.), here's a quote from the general manager of Nub's Nob in a magazine article several years ago (paraphrased):
"I don't care about the mints on the pillow, the temperature of the pool, or the snow hurting the greens on the golf course, because we don't do those things here. Our focus is 100% on the snow quality."
That sums up most Nub's devotees right there. The area is undoubtedly all about the snow. You have skillful grooming before and after the hill closes for the day, and more-than-adequate snowmaking (average of one snowgun per acre of non-gladed terrain, which equates to around 240 guns). Yes, it's a day ski area, and a really really good one. If you're coming to the area for more than a day, you'll need to stay at a hotel Harbor Springs/Petoskey, or some of the condominiums nearby (there are a few condos that are slopeside). Not much to be had for nightlife, but so what? If you're really serious about the sport, you're probably too tired to party much after a day on the slopes. I didn't give a rating for nightlife as I'm not a participant; can't really judge what I don't partake in.
The trails are a lot of fun. The several black trails are aptly rated for Michigan standards. Lots and lots of blue trails, and quite a few green. All but one of the glades are double-black rated and they'll keep you focused. One very minor gripe is getting to Pintail Peak. If you don't get off to a good fast start, you'll need to skate-ski to make it there. Some people would complain about no high-speed lifts. In theory, it's a nice idea, but it would just add more crowds to the hill. No thanks. I didn't rate the terrain park as I gave up jumps and tricks many years ago. That being said, the park looks like a whole lot of fun; certainly popular with the boarders and freeskiers. The Superpipe is pretty huge.
Both of the Boynes have higher verticals and more acreage, but their snow quality isn't always up to snuff, and you tend to see a lot of people spending more time posing for photos and hanging out in the restaurants/bars than on the hill. Nub's is all about the snow, and they do an amazing job. Period.