1. Not while leveling although I doubt there will be many pugs for the high end operations (8man raids)
2. No they dont disapere you can do them whenever you want.
3. What was so complex about WoW instances? Really? Move from crap, dont stand in crap, hit boss before he craps on you. Every single 5 main since Burning crusade heroics have been faceroll.
So far I have done the first 6 instances and they have all been very good fun all provided a challenge the first time I entered them. There are many of the instances on you tube and there are dev walkthoughs on the media page of the site, so check them out and make your own opinion.