I know when I went to look round schools to work in that I would get 'a feeling' or there was a definite 'positive atmosphere' IYSWIM.
I also think that, yes there will more than likely be stuff on the walls, but if you take a closer look, is it all work that belongs to the children, or is it possible that the teacher has just printed off and stuck bits on to look pretty? To me, that would be important.
There are lots of things I think that go towards making a happy school - pleasant, eager TAs, dinner ladies etc. Evidence of outdoor space actually being used - the amount of times I went somewhere that boasted an 'eco garden' or the fact that they have a pond etc and yet when I asked the children if they used them, 7 times out of 10 the answer was no.
I always think a school that encourages interaction across the year groups functions much better as a community than if the year groups are kept separate - so I would be keen to find out if, for example, the year sixes are encouraged to play nicely with or complete projects for the nursery children/lower years.
Do they have lots of speakers in? Do they encourage parental involvement or do they actively discourage it? One school I visited had a 'no parents unless they have made an appointment' policy. The headteacher said that no parents were allowed to accompany their child into school in the mornings and there was a time period of twenty minutes at drop off time in the morning where staff were not allowed to answer the door! If a parent did ring the door bell to try and get in, staff were not allowed to 'advertise their presence' by walking past the door. In practice, this often meant that teachers were stuck in the staff room for ten minutes instead of taking the register...
Sorry, I realise that is rather a lot there, and most of it will probably be impossible to see/ask about on a short visit, but they are things that I think I would be investigating if I were looking for a school for my DC!