I think that part of whether or not its safe relates to who many other kids are trying to use the slide at the same time. I work at school, and you can only play on the slide by going up the ladder and down the slide (on your bottom, feet first.) It might seems like overkill, but we have kids get hurt pretty much every day in one way or another on the playground, so we've ruled out the actions that caused the most injuries as well as the ways of playing that resulted in others not being able to play.
Swings can only go back and forth in a straight line -- no going from side to side.
Since all the children follow these rules, more children can PLAY and fewer end up in the nurses office.
When my kids were little and I took them to the park, and there were just a handful of kids there, I let them do whatever they wanted. It felt really odd when I first started enforcing the rules at school, but now I've spent enough time on the playground -- when 100 kids are being monitored by 5 adults -- to see the reason for the rules.