I tend to ask for content as soon as possible, largely because most customers are very slow to come up with it, so if you don't ask for it on day one you won't get it ahead of deadline and this will somehow be your fault in the eyes of the customer. It's annoying but it's how the world works.
I find that adding the initial content myself is a good way to verify that the site is working the way I expect it to and it gives the user something to work with when I hand it over. In my experience that extra practical testing and the amount of hassle you avoid by not expecting them to understand all your paradigms from the very start is well worth the time cost of a bit of cut-n-paste work.
Prior to content if I'm showing things off I use the Lorem Ipsum text ( grab a bunch from Lipsum.com ) as an obvious placeholder, same as most designers seem to.
In terms of adding Features I don't like to give them any new type of thing that isn't there at the start unless basic they ask for it directly. So if they decide that they want a slideshow I want them to come back to me and ask for it - I may have it ready and have no problem integrating it ( indeed I should do those things if it is an anticipated feature ) but that extra bit of work involved in coming back to me means we can make sure it does exactly what they expect, make sure it fits with the rest of the design and - importantly for day to day life - justify a maintenance agreement.
Once a feature is there on a site, unless its something that is likely to introduce unwanted complexity for users, I would aim to set it up so they could add more of the same kind of thing by themselves. But for the first one, I like to be in control ov that.