Is it possible to filter by level? (e.g. only show parent items which meet the other filter criteria?)
Unfortunately, that's not possible, but we may consider adding such option in the future and you're welcome to request it in our Votebox: http://todoist.com/Vote/
+1, I want this too please.
Filtering really is the weakest, yet one of the most important features in Todoist. It's terribly inconsistent and looks like it's parsed with regex which is really the wrong tool for the job.