As a seasoned cottage-renter...
Milk, tea, coffee (ideally "real" not instant, with a cafetiere), biscuits, salt, pepper and olive oil in the kitchen. I would include welcome pack in the price - if I'm renting a cottage I will pay significantly more for one that looks lovely, but would be a bit annoyed to be asked to pay a few quid for milk and biscuits and shampoo. Irrational but true.
Easy to clean
Provide all towels, bedding and some toiletries.
Cotton bedding and duvets if possible, not blankets (urgh)
Nice garden/place to sit outside if possible
Mirrors in sensible places eg over the sink in the bathroom
Make sure all furniture is secured to the wall, heavy TVs secured etc to be safe for small children.
Smoke alarms and carbon monoxide alarms
Info pack with notes about the cottage (eg how heating works), local taxi/emergency numbers, leaflets about local restaurants, attractions etc.
If anything is wrong eg broken door handle, blown lightbulb etc., make a priority of getting it fixed - don't leave it and don't get shirty if renters mention it.
Be flexible with nice renters. I really get upset by incredibly suspicious and accusatory cottage owners who automatically assume I want to sneak 45 people into a cottage that sleeps 6, and intend to leave it in a mess. I don't. I know there must be some renters who abuse cottages but I think some cottage-owners get into a mentality that all members of the public are bastards out to rip them off - don't be like that! If people are nice and responsible, then let them bring an extra readybed or stay a couple of hours over as a special request. Being nice will bring repeat business.
We recently went to the cottage of our dreams (sadly only for one night as we were on a weekend tour) - the owner was lovely, she allowed us to bring readybed for DS, asked how old he was, greeted us with a present for him of some chocolate (ok not all parents would like that but she checked with us first), was available but not intrusive, cottage was beautiful, well-kept and everything provided.