The only think I'd say is to make the "update" button more prominent (eg add another one beneath the "searching for tags" sliders). A few times I wondered why it was not updating the results, and had to hunt around for it. Maybe some people, at the margin in terms of interest, would give up on it because of this.
It would be nice if there was some natural way for the results to update without specifically requesting it - eg. like how the "moreofit" links work. The way google does it in "search tools" for time choice is interesting: it provides typical defaults, that are linked (eg. Past 24 hours,Past week etc). I've always used it by just clicking on one of them, so the issue of "updating" doesn't occur. [reminiscent of 37signals' defaults]. Perhaps the truth is that while someone might theoretically want the results from the past 22 hours, in practice no-one ever does. Thus, the defaults are adequate. For your importance sliders, I guess this could be done with two links "important" vs "secondary" for each slider.
Of course, sometimes you want to assemble a search, and only then execute it (not have it update after every single term is added). For this, the standard practice seems to be to have a "search" button directly underneath however the search is configured (eg this what google search does for "custom range", for time, in web tools). So this would mean adding a "search" button beneath the sliders (not labeled "update", even though that is also an accurate description - I guess, maybeuse "moreofit", if you want to promote the brand at possible expense of usability).
tl;dr add a "search" button beneath the "searching for tags" sliders.