Well, the average ten minute song doesn't have ten minutes of song in it, know what I mean?
Let's look at Metallica. On Ride The Lightning, songs are between four and six minutes, with maybe one seven or eight minute one. Almost all of these songs go for a perfect length, and don't where out their welcome.
On Master of Puppets, Hetfield seems to have gotten it in his head that longer songs are more, I don't know, intelligent, or progressive. Regardless, songs will start, finish, and then repeat themselves a whole fucking lot until you've got yourself a nine minute epic from what should really be a five minute song. The problem was even worse on ...And Justice For All, there's a song called To Live is To Die, a nine minute song with eight lines of lyrics. Now, I like instrumental stuff, but that song was far too repetitive to be an (almost) instrumental.
In-A-Gadda-Da-Vidda is my favorite long song, because it changes. Two minutes of singing, a few more minutes of all the instrument dropping off except the drums, a few minutes of the best drum solo ever, a few minutes of all the instrument rejoining the song, and then a few more minutes of singing.