Moving on from UmmAandY's somewhat unrealistic (for UK) Nursery ideal......
It depends a bit what you are after. For childcare, some things become more important than if you are just taking them there 3 mornings a week for socialising and interaction.
Then, you need things like opening times that fit in with your work hours plus commute. You need good communication - for shift changes, and practical things like parking make life so much easier when you are tight for time / stressed at either end of a day at work.
I hope it goes without saying that all Nurseries need loving, caring staff. There is always going to be a balance between what we as parents are willing / able to pay, and what would be ideal in terms of additional staff.
I do think having lots of natural light is important to me (you'd be surprised how many Nurseries have dc in rooms without windows 
), and lots of opportunities to be outside / use outside space. I also think it's important to have lots of space inside, as, I don't care what anyone says about forest schools and 'no bad weather only wrong clothes - no-one really wants to be outside in sleet / fog / driving rain etc for 9 hours a day, 5 days a week.
Good food prepared by a properly trained cook is a real plus point, rather than expecting one of the staff to pull something together.
I love a Nursery that is really inclusive, but also knows how to support the dc properly and find the additional money it takes to support all those additional needs.