The trouble here is actually bigger. Right now, you can easily save an empty filter: no title, no pattern, no selected options, anything. Of course this makes no sense. I believe we should mark as required some basic fields, i.e. title and pattern. My reasons: title is needed since it's the only public description available: people would have no idea of the broken rule. As for the pattern, it would make no sense not to write one, since it's the core of the filter itself. I'll see if I can make a commit to fix this.
BTW I'm quite worried for this, since when Ace Editor will be implemented, we will almost surely need some JS trick to make the editor required: being a DIV element, it cannot take "required" property, and by putting a "required" on the hidden textarea the error message won't show (right because it's hidden).
@Huji: the "required" attribute warns the user when they try to submit the form with empty values by showing localised error messages (and prevents submitting the data). This is probably enough for someone who forgot a field. However, as I was saying above (which I wrote before reading your reply), it would probably be better if we build an independent validation system, which we may flexibly use with Ace div once ready.