I'm in agreement with this statement. *Disclaimer: I know other metal genres have been mentioned, but I'm going to go into more depth of why they need to be mentioned.*
On that note, in the category of metal, it's very, VERY lacking. For one, only listing Metal-core really perturbs me simply because there are other genres of metal, and while metal-core is very well known, other parts of it really need to be included considering how vast it is (I could go on a rant of Metal-core, but I will not). I agree that Progressive Metal is a must up there, such as Dream Theater, Opeth, and Rush (I know there are more, those are just some of the more popular ones).
What shocks me more is some of the very well known Metal-core acts like Lamb of God, All That Remains, and Killswitch Engage are not even up there, when they give some of the better representations of Metal-core. Very shocking, considering how popular these acts are, with Killswitch Engage being the most popular, and All That Remains/Lamb of God fighting for other top spots.
Also, different metal genres like Melodic Death Metal, Power Metal, and Thrash Metal are a must, as all are different from the current metal genre as well (I could list more, but I'm just sticking to what's well known). In the Melodic Death Metal genre, bands like Amon Amarth, Arch Enemy, and At The Gates are very well known, and good examples of Melodic Death Metal, and differs greatly from Metal-core (In Flames could be cited, but they're not currently Melodic Death Metal). In Power Metal, there are examples such as Ronnie James Dio, Kamelot (ex:
http://www.youtube.com/watch?v=3wRreoNRGwk), and Blind Guardian (ex:
http://www.youtube.com/watch?v=kVNh_1vEgJI), all three of these bands being very well known for their genre. Then, there's Thrash Metal, the easiest to recognize with bands like Metallica, Slayer, Megadeth, and Testament being some of the more recognized Thrash Metal bands. All of these genres have very different characteristics from Metal-core.
Heck, Heavy Metal is different from the metal genre listed up there, and with that, bands like Iron Maiden, Judas Priest, and the already mentioned Zepplin (Sabbath is up there, yet in the wrong spot).
While I'm at it, where is Progressive Rock? It is a very well known genre that deviates from the rest of the genres listed up there. Pink Floyd, at the very least, needs to be up there, and Rush can also fit into this category, for two well known examples.