I think the main thing is that the facilities must be clean. In France there are those squeegy things so that you can clean the shower after yourself. It's a great idea and works. I hate it when there is nothing to use to clean up after myself.
Underfloor heating in the shower block is great. At the campsite I'm thinking of it had underfloor heating and a code to get in and out of the shower/toilet block. the door handle was quite high up and young DC were asked not to go in on their own. This meant the facilities were kept very clean.
I love Slubber's idea of two draining boards at the washing-up sinks. I've always thought that, too. Make sure you have enough sinks for the number of pitches.
Please don't keep caravans and tents separately. We are all people and it would mean that my three DBs and I couldn't take our families camping together, as two have tents, we have an FC and one DB has a caravan. We are camping together in July. Also, we are camping together with a big group of 8 families in June and we cuoldn't do that if tents and caravans were kept apart.
Leave a big space in the middle of the field for people to play games. Last year the big group that we go with played rounders and other people we didn't know camping on the same site joined in. We were pleased.
Please don't overcrowd the site on a Bank Holiday. We have a couple of favourite sites near us, but one of them really crams them in on a Bank Holiday (into the play space in the middle of the field).
A bit of recycling is good. Different bins for different materials. On one site the bins are in an area surrounded by trees/hedges. It's a really good idea.
Small shop/cafe on site, but no club house, please.
I think that's about it.
Also, but it's difficult to deal with, don't make people go inside their units on the stroke of 10.30 pm. I think if the units are far enough apart a murmur of voices after that time is perfectly acceptable. Obviously no music or anything like that.