How it runs should be your top concern. While you can manage many other things, you cannot change how good it is. You should make sure that it will run perfectly fine for you at all times. Regardless of weather or time, you should be able trust that your HVAC system will be in working order when you need it. If it is not, you are going to be dealing with the temperatures outside on your own. That is simply unneeded, especially since this is supposed to be there for you.
Having constant maintenance done on your HVAC system is a bother. It is going to end up costing much more than it is worth and it will become more stressful than you expected it to be. When you combine the two, you have something that is not worth what you are putting into it. That is why you have to consider reliability. You should be able to use this over a long period without failure or issues. If it is something that you can trust to work and that will do what is needed, you will be able to make use of it. You just have to make sure that the brand and option chosen are going to be there for you.
When you do have to do maintenance on it, it should not be overly difficult. There are certain tasks that you should be able to do on your own, without the assistance of a professional. During the more complex fixes and maintenance, however, it should not take too much time. This is something that should be easily managed without too much stress involved. You should be able to take care of whatever comes up.
Usability is another point that you should keep in mind. If it takes too much time or effort to get everything working, you are not going to be able to take advantage of it as soon as you need to. This can get in the way quite often, even leaving you uncomfortable as you try to get it to start. This should come easily to you, regardless of your knowledge concerning an HVAC system. You should be able to navigate it and have it doing what you need quickly and without a lot of effort put into it.