I wish I could select "between 2.5 and 6 minutes". That search can translate to 2 queries to YouTube (<4 and 4-20), then the results can be combined and pruned to keep only those between 2.5 and 6. To get enough videos if there aren't enough after pruning, we could access the 2nd, 3rd and so on pages from the results. But I doubt YouTube will like 6 searches in a row.
Yea and I'm doubtful we'll see a service willing to do their own post-processing per-query while also being at the whim of Youtube's API (official or not).
Ultimately, I would like these features to come to Youtube itself since there's a lot of nice features built into it that would be hard for a third-party to replicate without permission (such as playing videos inline on hover, with captions).
I doubt it will ever happen. This is Google after all, not a small company we can hope will get it right after a while. They've left the search parameters shitty for years. Google, arguably the most advanced search company ever, can't make an efficient filter for custom time ranges? They obviously can, but, as other comments have noted, they seem to think that good search is an anti-feature.