I've returned to give a bit more info from further playtesting.
First, I want to cover some bugs I've discovered. First, if you save while people are at their table but have not gotten their food yet (I think. It could be if they're eating already) and then load that save, the customers affected will suddenly get up and leave, giving you negative ratings. Also, if you save while a server is cleaning and then load that save, you'll lose the server. I also had one instance where a monster chose to attack and destroy a wall instead of going through the door despite the door both being open and being closer to where the monster came out.
With that said, I now have a better idea of other aspects I feel should be tweaked.
First, I'd like to suggest that the starting position for the diner be one tile forward. The reason why is because it feels rough that if you try to do ANY sort of expansion of the kitchen (which is desperately needed), you'll open a second pathway. However, you can still expand the dining area down one tile without risk.
Another suggestion would be to slow down the rate that employees will start demanding significantly higher raises. So when you're starting off and hiring your first server, Jonas is the most obvious pick based on his skill level and current state of happiness. However, his starting rate causes him to lose 15% happiness per day a little more than a week after you start. Since this happens very shortly after he had a huge drop due to the monster attack, it means you have to jack his pay way up. By the time you reach the end of the demo, you're paying him nearly 50% more from where you started, which feels like a lot.
I'm also going to mimic the recommendation of another and to suggest reducing the starting amount of money.