yea, this is probably the case. All metal, whether it is napalm death or symphony x is forceful/aggressive.
Not true, some bands focus on incorporate soft melodies and don't focus on being aggressive or forceful as other genres, one example is Agalloch, specifically the Mantle. 75% of the guitar playing was acoustic, the other 15% were focused on two songs, which featured some folk acoustic breaks within those songs, and some light riffing in a few songs.
Also, another example to take is Funeral Doom, which focuses on being melancholy, brooding and slooooow as fuck instead of being aggressive and fast. The same could be said for Doom as a whole, but Doom has an element of aggressiveness in it that i'm tentative to include it.
Now, elaborating on an earlier post I made, I feel that with Metal that there's a certain atmosphere about Metal that kinda makes you feel it is Metal. Of course, this is good and bad, as you know myspace scenesters will start considering My Chemical Romance Funeral Death/Doom Metal any moment from now. However, what makes Metal metal is entirely up to your choice, and it helps that we have sites like Metal Archives (and probably Ultimate Metal) to help steer us in the right direction.
So, long story short, Metal is what you pretty much make of it. There isn't really a defined sound to Metal (other than guitar distortion, but even then that's used for rock bands, so i'm not entirely sure) just the defined sound of it's subgenres. Some people choose to consider Static X and Linkin Park metal, and others choose to call Behemoth's earlier albums black metal and their later shit Black/Death Metal. It's all about opinions.
I might be wrong on this, and if I am, then that's fine. I don't consider myself to be an all-knowing metalhead, i've been wrong several times. Until then, i'm just perfectly happy using sites like MA to help define the metal bands i'm listening to.