Just quickly, off the top of my mind, I can think of these
How fast will Wikipedia continue to grow in the near and far future?
Is there a limit to the growth (as per the logarithmic growth model)?
(...Or is "To know all is not permitted" !!?)
How will Quality of Content be affected in the near and far future, as Wikipedia grows?
Is Wikipedia truly Reliable? Would you bet your life on a Fact from Wikipedia?
Can the regulatory mechanism be improved? How?
Do "Editing Wars" always lead to the "unbiased Truth"? (Can the Truth itself oscillate?!)
Does the process of "finding consensus" always lead to the best entry? (Is the "Average" always the right answer? Are there cases where the 'populist decision' may not be the 'right one' ?)
What motivates Contributors? (The question "What motivates Seekers?" is fairly trivial)
What is the "carrot" at the end of the stick for contributors?
Will a contributor contribute content that has a high personal cost (or opportunity cost!) associated with sharing? (e.g: A stock trader chancing upon and then disclosing a piece of positive/negative news about a listed firm before it has broken on any other news channel, and without profiting personally, Or an inventor publishing on Wikipedia without thought of personal gain from his/her invention)
(I use the term very loosely here, as the "sum total of all human social knowledge".)
Can one uncover hidden cultural facets by studying the topography of the wikipedia network, by observing clusters, by deducing from "association" something of value? (somehow, again, I think - "Meme Theory"!)
What is the contribution of Wikipedia to Culture?