While leveling up bagspace is usually at a premium. It may have been my imagination but it seemed like a lot more quests were giving rewards than they did when going from 60-70. Most of it isn’t worth hanging on and unless you can disenchant it, your only options are to vendor it or let it take up bag space.
But not all vendor trash is created equal. I put together a list based on level 79 greens of which items fetch the most at the vendor. The general rule is that weapons vendor for the most, then plate, then mail, then leather, then cloth. Within an armor class, legs are most, then chest, shoulders, helm, boots, bracers, hands and finally waist. Instead of clogging up the front page with a detailed list, I made a post on the forum with a more detailed list: http://www.tankhard.com/smf/index.php/topic,206.0.html
I also highly recommend the addon AutoProfitX (http://wow.curse.com/downloads/wow-addons/details/auto-profit-x.aspx) which puts a button at the top of any vendor window that lets you automatically vendor all of the grey items in you bags. You can also set it up to vendor selected white items as well (I am looking at you fish scales and fish oil).