Staff turnover is important both because it reflects on the staff, management and nursery as a whole, and also because once your child is settled with a key worker, you really don't want them to quit.
Free-flow is good, but watch out for true free-flow vs 'we push back this barrier in the afternoon and that's about it'.
How child-led are they?
Personally I like mixed age group key families, but these are rare.
Safety - I turned up at one nursery for a visit and could have let unsupervised children out into the road. Horrifying.
Look around middle of the day, what are the children doing? Do they all look engaged in what they're doing either being led, facilitated or just exploring - or are they sitting on their own, unengaged and staring into space. Are there staff cuddling children?
Naps - are there set nap times (bad) or child-led (good)? Is there a nice nap room with a range of sleeping beds - cots, prams, nests etc, or is it plastic covered mattresses the hall floor?
How much time do they spend outside? The more the better.