After putting in your character and realm, you get a list of all the gear you've got equipped, matched up against a list of all the gear available at the item level you're looking at (you can customize exactly where the item levels come from, if there's a site whose estimations of the gear you value more), with yours highlighted. So basically, you get an up and down list of where to go from the gear you've got, and you can easily see where it all comes from and what kinds of stats it gives.
Of course, this is still more of a guideline -- just because gear appears above yours on this list doesn't mean that it's better than yours for your class and spec. And attainability is a big factor as well -- if you're not in a raiding guild, your time might be better spent grinding rep rather than trying to suffer through with PuGs. But as an overview of the gear available to you, Gear Wishlist works great. Hopefully the site will stay up under our linkage, and if it doesn't, check back in a few days to see if it's slowed down.