Phil W and Quizotic Geek mention the path has to have a good surface, and not be covered in ice in winter.
In the Netherlands I can confirm that following snowfall this winter, the bicycle paths were cleared of snow in the wee small hours.
I ventured out at around 7am one weekend when there had been a snowfall, and the cycle paths were all cleared by what I think are the little service vehicles which the local council uses on the paths.
The interesting thing is that the cycle paths were all cleared, before the roads were touched!
And this was no haphazard badly done job - all cleared of snow and ready to ride on.
One other thing which stops cycle paths being ghettoes is priority at side roads and around roundabouts.
Dutch drivers are trained to look out for cyclists during their lessons, and are cyclcist anyway. The cycle paths continue straight across side roads, where the 'Give Way' lines are set back.
Cars will awlays wait on the main road as you cycle across a side road. Try that in the UK - instant SMIDSY
Also on roundabouts, you might go round the roundabout on a separate lane, but once on there you are judged to be part of the traffic. So you just sail pn and expect the cars entering the roundabout to give way, which they do. Again try that in the UK...