If you are a successful and happy Mastiff owner, you may already be up to a Great Dane! And probably know much more than I do.
Short lifespan is a big one. Along with their health/medical/nutritional needs. Plus their many needs to comfortably accommodate their overall size – I figure their special cushioning for beds probably occupies as much or more floor space as a normal-sized couch/sofa. Some people might need a larger vehicle as well! I’ve heard that they are wonderful dogs, but accommdating one and keeping it healthy and happy is a job as big as a Dane.
You may already be well able for a Dane! That’s just the elevator version I remember that rules them out for me as a dog owner. I’ve never sought personal experience with Danes.