I have attended Culture Night every year and thought this was the best year yet, and I was one of those drinking in the streets. I lived in Madrid for years and Culture Night very much reminds me of any of the district fiestas which had a very similar set up.
The balance of family orientated events during the day and a more party atmosphere in the evening was perfect, and I personally did not see any anti-social behaviour other than people drinking.
Two big improvements which are needed, in order to tackle some of the issues that have been identified in comments, are portaloos throughout the city centre so that people are not forced to pee in the streets. I went in to the Oh Yeah Centre to use the toilets and they were queued to the main bar, which was ridiculous. On the walk back from Biggest Weekend and AVA earlier in the year I was really impressed that both had dotted portaloos on the exits from the venue and along the road - this is responsible and should be enforced for all events.
Secondly, they should have further bin provision. I brought a bag as I hate rubbish being thrown on the ground, but a lot more bins were required for that amount of people being in the city centre all at once.
It's great that we have large, non-sectarian and inclusive events such as Culture Night, I have thought for a while they should do an event each season, but from memory they nearly lost their funding last year or the year before so I'm not sure if they could fund more events.