It's time to set some things straight about Nu-metal, like what IS nu metal and what is not. First off, Nickelback and Three Doors Down are MUCH more like rock bands, not metal. Granted they suck, but they're not metal in the least, either lyrically or in theme. This is important because calling bands like them "nu-metal" gives nu-metal an even worse name than it already has.
I also notice that a lot of people here are jumping to the defense of System of a Down and I must say I agree. SOAD is the only "nu-metal" band that really stands out in the category. System's songs and lyrics have some cool themes much like early Metallica (no, I'm not saying they sound very much alike but the themes of the songs are similar) unlike the emotionally insecure screaming of Slipknot (they have 4 more members than they need and Cory can't sing for his life), Linkin Park (Chester sounds like he has a vibrator stuck in his larnyx) and of course that poser Fred Durst. SOAD's songs are also quite catchy 99% non-depressing, they'll even make you laugh sometimes. It is for this reason that I find it hard to actually call them nu metal. Maybe Alternative Metal would be better for SOAD. Besides, does their lead singer Serj sound much like your typical screaming nu metal frontman?
Overall, nu metal is like any other musical genre, with good bands and bad ones to match. Good nu metal bands include SOAD, Atreyu, Disturbed, Ill Nino, and some KoRn. Bad ones include poppy bands such as Linkin Park, Slipknot, POD, Limp Bizkit, some KoRn, Puddle of Mudd, and Drowning Pool.
Why diss nu-metal when you could be dissing rap instead?