Sorry, sf7307, but that's a joke on naive newbies.
All casa particular rooms are required by Cuban law to have ac in order to be licensed.
"a window to the outside or to an interior open-air courtyard", in Havana Vieja at least, could mean looking out on an air shaft, which most of the closely built structures there have (think NYC Lower East Side), as easily as it could an actual courtyard or balcony. Casas in the Vedado neighborhood of Havana, which has more large houses than apartments, may have more open area. Most of the casas in smaller cities/towns that aren't as built up may be closer to having an actual terrace area. Vinales casas may have a roof terrace to see the views.
"Modern imported mattresses" doesn't say imported from where (former Soviet Union? China? Vietnam? I don't think there is a domestic industry producing mattresses.) or define modern (maybe not old, dirty and smelling of mold?), and are likely to be the same one piece, feel-the-springs-to-some-extent type every other casa and Cuban individual has. If it were possible to import a mattress and box spring of the type we are used to in the US, people with money (in the tourist trade) would have them for themselves too. Were you expecting Temperpedic?
Many (most) casas considered "standard", and which charge a "standard" price of 20-30 CUC have ensuite bathrooms and hot water.

Honestly, it's only the A/C I care about. I hope it works, so I can sleep.

When is your trip?
In September, ac was definitely a must.
In March, a fan was enough.

Note: Some of the ac units are old and loud, and if left on for a prolonged period, smell of diesel for some reason.

My main problem for a decent night's sleep were beds that I could feel the springs through. (Princess and the Pea for sure. LOL) Hope you have better luck.
Also I still got mosquito bites at night in certain locations (not Havana), on my face, the only part that was exposed. They'll tell you that the cold of the ac keeps them from flying, but there are no window or door screens in casas in Cuba.

SC - though we were not over-impressed with some of the casas we stayed in, you seem to have been particularly unlucky. None of the a/c units we came across smelt of diesel [or anything else] mattresses were at least adequate [i don't remember any springs sticking into me though some of them were not very thick], none of them were mouldy and all of the rooms were clean.

sf - i agree with SC about this "every Comfort room has air-conditioning, an en suite bathroom, a window to the outside or to an interior open-air courtyard, modern imported mattresses, and cold and hot water." - those are all required of every casa room anyway and we never saw one without them. Nor is the price necessarily a guide to the level of comfort as some casa owners choose to keep the price low in order to ensure maximum occupation. This is because they pay tax on the rooms, whether they are occupied or not. You will also find them very keen to feed you, not surprisingly really because this boosts the income from which they have to pay those taxes. Casa meals do tend to be a bit samey - rice and beans [Moros y Cristianos] with chicken, pork or "lobster", and salad. This is not a criticism of them, it is what they think tourists want. for something a bit more adventurous it's worth finding a Paladares [private restaurant] or trying out a hotel dining room, though they are very mixed. The very nice and comfortable hotel we stayed in in Santi Spiritu served terrible food, but the one we tried in Remedios [where were we staying in a casa which BTW had a lovely garden for us to sit in] was great and we ate there every day. The Paladar in Remedios was very good too though we were overcharged for the wine [always ask for the price before you buy, however nice the person serving you is].

In short, you never quite know what you are going to find, but that is part of the fun of going there!

annhig--You must have a younger body/more energy/more resilience and much more patience. Actually my camping tent is furnished more comfortably than some casas. And also, never encountered these problems in Mexico or Brazil or Central America at a similar price point. Sleepless nights (from bad mattresses, hard neck-breaking pillows, and ancient sheets with pilling that acts like sandpaper), insect bites and poor food have ceased to add up to "fun" for me. They do take their toll on me. I love the people I've met, and have formed ongoing relationships, but I can really take only a few days in Cuba before I'm ready to head home. Am considering only long weekends from now on.

annhig--You must have a younger body/more energy/more resilience and much more patience>>

perhaps we were just luckier, SC.

SC - How about considering a hotel? Our beds (and towels) weren't heavenly, but they were fine. Completely comfortable otherwise. Still had plenty of contact with the Cuban people.

crosscheck - the one hotel that we stayed in [in Santi Spiritu] was very comfortable with better linen and towels than we had in the casas and even a TV! Much nicer than many in Havana from what I have read. But we enjoyed the casas as well because of the contact with cubans and other travellers as well, which wasn't as easy in hotels [at least not the one we stayed in].

A mixture might be a good idea.