1. Drafty areas need to be caulked, spray foamed, boarded, or somehow covered. The last thing you want is you chickens to be shivering from a cold draft hitting them. This can cause all kinds of illness in your flock.
2. Proper venting is super important if you have to little the coop will become humid, high humidity can cause Upper Respiratory Infections. As well as it can cause frost bite on the chickens combs and other sensitive parts.
3. Heat? I do not heat my coops. I tried it one year and we almost caught the coop on fire never did it again. We do however use the deep litter method to add some warmth/insulation into their coop. It provides a bit of heat in a safe way.
4. Fresh Water, this becomes more difficult depending on the climate you live in. For us we freeze a lot. So we are using Harris Farms Poly Plastic Drinker on their Heated Base this year. In the past we have tried many other heating systems some worked ok some not at all. Either way we always ended up wet, cold, or worrying about if we going to burn the coop down.
5. Roost bars are essential in keeping your chickens warm and comfortable. We have learned over the years to place good sturdy roost in. We use 2x4's we place them to where they give the chickens the widest spot to stand on. This allows them to lay securely snuggled in while being able to lay upon their entire feet to keep them warm and toasty. We do have a few girls that choose not roost at night for them we have milk crates filled with straw for them to sleep in.