In short, three key factors: accessibility, functional and emotional component.
Accessibility: How easy is it for your audience to do what they need to do?
Functional: Does the experience deliver what the audience need it to do?
Emotional: How the experience makes the audience feel?
The most basic of the three is the functional component. This is what the audience/customer takes it as given - e.g. a clean room for a hotel stay, appliances that work, meals that don't cause a tummy ache, hygiene practices, etc. Meeting this need is the minimum, but it does not set you aside from competition, not at all.
Accessibility is important because if the process, the usage or getting through the helpline is cumbersome, it means the experience is not seamless. This equates to wasted time, dissatisfaction and frustration on the part of the user. Make things EASY to use, to enjoy. Are your printed or digital instructions easily understood? Are the font size too small? Are the bathroom amenities within easy reach? Are the room's power points sufficient and well positioned? Is your membership sign-up off-putting and can the staff explain the benefits clearly and succinctly?
Emotions, ah......this is the secret ingredient of a great experience. It is that almost inexplicable feel-good factor, something that makes you want to share with and rave about to your friends and family. If, at the end of the service encounter or the hotel stay, the client is asked, "How was the service?" or "How was your stay experience at XYZ Hotel?", and he/she responds with a lacklustre "fine" or a mere "alright" with no specifics, chances are, there was not anything in particular he/she could recall (and retain in memory) nor give a positive shout-out to.
So, what type of experience is your organisation/business seeking to deliver?