A further suggestion on filters:
I like how Qantas has the "lowest price around the dates" or the flexible dates calendar which shows you the cheapest flights available on that day.
However, filters (time of departure, no mixed class bookings, operating carrier, non-stop) should be available throughout the fare search process, and the "lowest price" should update to reflect this.
For example, say I search for a BNE/SYD flight. I get the usual fare display with tabs at the top showing the lowest prices on +/- 3 days either side of my selected date. Let's say the Jetstar operated flights are the lowest on each day, at $112, whereas the cheapest Qantas operated flight is $125. If I choose to apply the filter to exclude Jetstar operated flights, then the tabs which show the cheapest price on that day should also update (i.e. they will now display the lowest price on that day
excluding Jetstar operated flights).
Matrix ITA search does this rather well in calendar view (i.e. do a flexible dates search over the next month rather than a specific dates search). When the calendar is displayed, the lowest price on each day is displayed. However, if you filter, say by airline, sometimes the filter will cause the "lowest price" on a day to be excluded, so Matrix ITA displays the next cheapest flight which fits the filter (the original "lowest price" is displayed in faded strikethrough).
See below is a sample BNE/MNL search, then the next one shows an example where I've set the filter to exclude Jetstar flights.
After filtering out Jetstar flights:
I have to give it to Qantas for the "fare vs time graph" which you can see when you go through the Domestic / International specials pages on the website. This is a nice graphical summary of prices when sales are on so you can quickly see when sale fares apply. However, it doesn't tell you whether all the cheap fares on sale are sold out on a given day (this graph works on date ranges, not specific days), and it also doesn't allow you to be more specific about the kinds of cheap fares you are looking for.