I think the main mistake most new greenhouse folks make (and it is natural) is that they assume since it is a greenhouse, and a greenhouse is supposed to be a warm, cozy place where plants will thrive, is to think an out-of-the-box greenhouse is suitable for year-round growing in colder zones (and I include myself when I first bought my greenhouse), when in reality they are only suitable for growing in about 8 months a year (extending the growing season about 1 to 1 1/2 months in each direction) without high costs for retrofitting with insulative materials and the crazy cost for the amount of heat that is needed.
I think the next mistake most new greenhouse folks make is thinking they can re-invent the wheel in accomplishing all this when there are tried and true ways to create winter growing conditions in a greenhouse that isn't designed for winter growing. (Once again, I include myself my first couple of years.)
Out of the box greenhouse companies are very misleading on both the above.
Keep asking questions; that's how we all learn about this addiction! In a few years, you'll be giving new greenhouse owners good advice.
