I answered earlier, but have now reread through the thread and later answers, so will reiterate the following;
Most Important is off-road parking (but also state whether this is only for 1 car, as you cannot assume that a cottage for 4, is going to be two young children, as older children or young adults may have a second car. So state carefully if the parking is one car only).
Wifi - Nowadays this is essential.
A welcome pack is nice, but mainly milk, tea and coffee, and maybe sugar. No Need for bottles of wine, scones and jam (which we have sometimes received). These things are nice but not essential. Please place the milk in the fridge. Yes, really, we once stayed in a cottage in Cornwall, where the owner left the milk on the doorstep. The heatwave of 90 degrees, meant that by the time we got there, the milk was off. The owner complained that we got there late (7.00 pm, from Kent).
I would not use any food that was left, except for salt, and pepper, so please check the freezer for unwanted items, as this is the place where stuff often gets left.
I would suggest you should know your market. For example, if the cottage is just for 2 people, then you would obviously not get families.
If the cottage is larger, specify whether cot, highchair, etc is available, and whether there is space in the main bedroom for cot, or travel cot, etc
For wheelchair users, specify whether there are steps, anywhere, ie steps between rooms.
Also, in one cottage, the bathroom was on a different floor to the bedroom, requiring negotiating an old spiral staircase, that was very steep, and probably not suitable for elderly, so this kind of thing should be specified.
I think everything else has already been mentioned.