Make sure the bed in the master bedroom is at least a kingsize, or ideally a superking - people don't want to be less comfortable on holiday than they are at home. Bonus points if the mattress is an expensive "dual zone" type or zip-link so that you don't get a slope down to a central well.
In the second bedroom, make the bed something flexible that can accommodate either a second adult couple, or two kids, or if there is just one kid then having the second bed able to be hidden away underneath to open up the room (if you are just one person in a twin-bedded room it is frustrating having a huge amount of floorspace used up with an unneeded second bed)
Provide a dishwasher - people don't want to be doing more washing up when on holiday than they are at home, and even if eating out in the evenings there can be a surprising amount of washing up from breakfasts, lunches and snacks.
Don't fill up all the cupboards in the kitchen with crockery, glassware and cookware - we need somewhere to store the food we need! I often have to start off a holiday let selecting the best cupboards to totally re-arrange, taking a photo of what it was like when I started, and then condensing 2 cupboards of kitchenware into one just so that I have somewhere to put food.
Make sure that the cleaner between lets is going to do a scout of the garden to pick up any dog-doings left by the previous holidaymakers before the next batch arrive (and obviously make it clear in your Ts&Cs that they will be charged for this service if they are so antisocial) - the next holidaymakers should not have to deal with another dog's doings, whether or not they have brought a dog of their own.