Bagged stuff is produced to a very specific price point, rather than to maximize pleasure. Basically, they grind crappy tea really finely to maximize the exposed tea leaf surface area, which uses less tea per bag to yield a cup that has equivalent color. It is difficult to get water to flow over whole tea leaves when they are packed into those flat tea bags.
Unfortunately, ground up tea maximizes bitterness while minimizing any other flavors that may have been present in the tea beforehand. Ground up tea also overbrews extremely quickly, making the flavor:bitter ratio even worse.
If you're shopping around at places that only sell bagged teas, Mighty Leaf and their ilk have pyramid-shaped tea bags and whole-ish leaves inside. These brew considerably better than Tetley/Lipton/etc. I would say they still don't compare to decent loose-leaf tea, but you can definitely tell the improvement.
When shopping for loose-leaf tea, it helps to find a single tea that you really like and then branch out from there. When I was a kid, I started off drinking mostly Japanese greens, with a preference for genmai cha (green tea with roasted brown rice). The tea quality used in genmai cha is generally quite poor, but it's still delicious. In college, I started sampling other greens, and eventually started approaching tea as a "tea omnivore." As you can see, I still have a pretty diverse collection, though I definitely have my preferences and can increasingly detect differences in tea quality within my own stock. Even so, I still drink a fair bit of genmai cha because it's delicious! In my case, "find what you like and drink that" means "drink tea erry day."
