A must for one bag travelers is the “bags” filter on Google, another one the others don’t currently let you do until you select the destination.
The duration and bags filters are the big two for me; because if a “deal” is going to tack on extra charges for a carry-on bag or stick me in an airport for 12+ hours; I’d rather not even see it.
The last one that Google allows you to choose before the destination and the others don’t is “Airlines” which can be great if you’re looking to use or earn miles.
There is one that seems common, and useful; this is “Stops”. I definitely like to limit the number of transfers, but at the same time it’s not really a major one for my initial search.
There’s really no getting around multiple stops for some destinations so it’s a factor I consider while narrowing things down.