As you start to search, it will suggest categories. It basically starts to pattern match against the existing categories. If you instead start typing "Analog to digital", it will show a range of categories as you continue typing. At the end of the phrase, it will land on "AD/DA Converter" (the proper category). As we get searches like this (and feedback about them), we will start adding to a list of colloquialisms and abbreviations that are common.
As for the filters, the data we have on that is actually split into serial vs parallel bus. These are found using the "Show All Filters" pulldown below the 8 main filters. Was it obvious to find thos?
As for the actual ability to find SPI vs I2C vs whatever, we can feed this back to our data people. I agree that sites like Digikey do a better job of calling out which protocols are supported.