For me, the three main aspects which account for a good passenger experience are:
Affordable fares: I wouldn't be feeling particularly "good" if I had to pay £200 for a return to Glasgow, for example. Having snapped up quite a few of Chiltern Railways' ridiculously cheap advance fares, knowing that I haven't been robbed half the GDP of the United Kingdom for a two hour journey, I can say it certainly does improve the experience!
Of course, though, it's all down to value for money. I'm much more willing to pay a lot more for first class on Virgin Trains (or now Avanti West Coast), especially if a meal is included.
Reliability: If I book a service due into London at 13:00, I expect to get there at around about the same time. I understand and I appreciate that this isn't always the fault of the operator, but I've had some absolutely horrifying experiences missing last train connections because of delays.
I was once forced to catch a Virgin Trains service back from Wolverhampton on a ticket that wasn't valid because Transport for Wales decided to cancel the last train as soon as it pulled into the station...
Customer service: Personally, I think that the customer service you receive paints a strong image about a company. Virgin Trains have always had top-notch customer service in my experience, and being treated well, with respect and a smile, really does have a positive impact on your experience.
I recently messaged a bus company on Facebook to enquire about fares, and their response - after almost a week - was "Email us at (email)". Wasn't overly impressed, and it didn't give a fantastic image of the company. I do appreciate that they actually responded to me, though, 90% of the messages and emails I've sent to various transport companies haven't been responded to at all since the start of the pandemic. Slightly irritating!