Do we really need a millennium-problem tag?
And if yes, what should go under such tags?
For example, I don't think either of the two questions (related to Navier-Stokes) currently tagged as such should belong to a millennium-problem tag even if the tag is useful. Those two questions concern elementary issues regarding the Navier-Stokes problem, which is rather distinct from the Well/Ill-posedness result that the Clay list explicitly asks for.
In the same token, I don't think an elementary question on Ricci flow should be tagged millennium-problem just because it is the high-powered machinery used in Perelman's proof of Poincare. And who knows how many questions we've already had on things tangentially related to the Riemann hypothesis, the B and S-D conjecture, or PvNP.