One of the biggest opportunities for improvement in audio almost across the board - amateur to professional videos - is the audio quality on videos.
For some reason it just seems to get ignored. I preach from this soapbox a lot. See my post Audio For Videos: Do Not Let Bad Sound Ruin Great Video.
Well, Sennheiser, maker of excellent quality microphones and headphones (see my review of the Sennheiser MKH 416 shotgum mic - my "desert island" mic), has been running a series of video tutorials on getting good audio quality on videos. See the first video below, where the talk about shogun mics like the MKH 416, as well as other kinds of mics good for video production.