I think that we should explicitly welcome open questions. 

I mean this, first, in a narrow sense. Namely, every mathematician hopes that the questions on which they are currently working---whether for their dissertation or a later project----is an open question. These open questions come in all types, some interesting, some difficult. Let us welcome them here and give them our attention and consideration. I suspect that with the talented people we have here on MO, many of these questions will find an answer. This is a sense in which I think that nearly everyone already agrees on the matter.

But second, in a wider sense, I think that it would actually be good for the site for us to encourage people to ask open questions of all sorts, including very prominent ones. Indeed, there already are some very interesting open questions, such as the question on polynomial bijections from $\mathbb{Q}\times\mathbb{Q}\to\mathbb{Q}$, which I think of as a highlight of the site, even though it is open. Let's build a collection of the best questions in mathematics! As answers, we can post information about resources, history, partial solutions, related problems.

I would expect that many of the more famous open questions could become some of the highest-voted questions on the site, a situation that would address the worry some have had about other softer questions currently occupying that role. 

Imagine that the top-voted questions on MathOverflow were all the most popular open questions in mathematics! That would be great.