Okay, I follow you now. Basically multi-criteria project searching (saving criteria to dynamic collections is incidental to the basic function which attaches to them). Of course, not a bad idea—something for well down the road as there are a lot of other things that need to be done as well—but I’m not going to contradict the fundamental assertion that being able to search for more than one criteria at once is useful. It is. 
But of course, practically everything you listed cannot even be searched for at all in Scrivener. That doesn’t mean it is impossible to get at that information. You could search for “Poem”, load the search results into Outliner, sort by date, select everything below 2009, and shunt that off to a standard collection. So just because you can’t search the date field for ranges doesn’t mean you can’t get range information (and arguably there isn’t a whole lot of advantage to having a UI that provides that, over the method I just described—total exertion is about the same—and the more fluid method that Scrivener provides allows for user tweaking to the results; static collections can be easily pruned or appended to whereas multi-criteria dynamic collections (smart folders, whatever), cannot).
Another trick is to take advantage of two concepts:
- Selecting the contents of a collection and hitting Cmd-Opt-R selects all of those items in the Binder
- Project search can be narrow down by the binder selection
And there you have a two-axis search. Not as nice as saving it for future use, again I’m not arguing against your point, just showing how you can get about doing the types of things you want using existing features. Rating would work in a similar fashion to date range selection. Presumably that would be a custom meta-data field, so search by Criteria 1 (“poem”); sort by Rating, and grab everything above “3”. If you want to use that compound result, save it a collection for future use, Cmd-Opt-R it, and do another project search.
In your particular case, since these particular metrics probably are not shifting around much, you could use static collections quite well. A dynamic collection so you can always quickly grab “Poem” and then perform search analysis to pick out ranges from it—might as well put everything from 2009 into a “Poems - 2009” collection because those aren’t going to be changing any time soon. Same could probably said of ratings. A static collection of 3+ rated poems. These dynamic and static collections easily become your search scope to work off of using the project search tool, instead of going back to the entire project every time from scratch.
Just some ideas—because frankly I wouldn’t expect multiple criteria project searching any time soon. I think it is on the list of long-term wishes, but right now there are a lot of big things to sort out.