I'm not really sure how much the OP has gotten into metal so far, but I must say I would be outright embarrassed to listen to most of the metal mentioned in this thread. This might be a difference of musical evolution, as I evolved into metal from hardcore punk, but I find Metallica and Nightwish to be intolerable.
Personally, the first band that got me into metal was Isis. They've got some great, punishing riffs and Aaron Turner has one of the best growls in the business. However, you mentioned that you didn't like the long-form structure of Tool songs, so maybe Isis isn't for you.
Some of my favorites:
Torche: Their early works weren't anything special, but their recent Meanderthal really showed their potential. The guitars are fast and catchy, and the vocals aren't heavy at all. I hesitate to call them ethereal, but for some reason that's the word that comes to mind.
Gojira: French death metal band. Fairly mild growling (i.e. you can understand what they're saying) some decent guitarwork and all their songs seem to be about whales. Can't really go wrong there.
Mastodon: As far as progressive metal goes, Mastodon has got it down. Leviathan is a must-have for any metal fan.
Kayo Dot/maudlin of the Well: Experimental metal hasn't really been discussed here, so I thought I'd give Toby Driver a shout out here. These two bands produce some of the most bizarre music I've ever heard, but if you can get into it, you'll love it. Pretty much impossible to describe in words.
Old Man Gloom: Another experimental project, OMG combines hardcore and doom metal inspiration into one awesome package. A few of their songs deviate from metal quite a bit; they use sound clips in a manner quite similar to post-rock bands like Godspeed You! Black Emperor.
Electric Wizard: Doom metal is another frontier that hasn't been discussed, so I figured I'd bring it up. If you're into Black Sabbath, I highly suggest you go in this direction, as this is the closest to what Sabbath actually was. And, to be honest, Electric Wizard is like Sabbath's secret, heavier incarnation. A lot of deep, droning, heavily-distorted guitar, but nothing too off-putting.
I guess that's enough suggestions for now. I really hope that the OP gets something out of this, because in my eyes the true scope and nature of metal isn't getting shown by the recommendations in this thread. However, I am not trying to start any type of argument here; I believe that metal is a very individualistic genre, considering the plethora of sub-genres that exist within it.
A bit of an aside: I'm definitely not into the whole satan/anarchy/hate aspect of metal mentioned earlier, and I think that shines through in these recommendations. Some of them are just fun, and the more progressive stuff usually has very introspective and deep lyrics.