While I'm not utter expert on rap/hip hop/whatever, I've listen to quiet a few selections of it. I can tell you that like Rock, no two rap categories are alike. That would be comparing punk with say metal. o.o;
[For simplisity sake, I'm categorizing different kinds of rap in chronological order, starting with the earliest. Also, some names may not be accurately portrayed. ...and for the last time, I'm NOT an expert! This is just my insight, from what I've heard over the years.]
(1) "Funkidelic Hip-Hop" (think Sugar Hill Gang)
This is pretty much a kickin' category of rap. One of the earliest, and still pretty good. It's got a nice flow, a nice beat to it. Since rap was just forming, their was no real 'attitude' attached to it yet. This scene is all but dead (though, Snoop Dogg did something for "Undercover Brother", I think... that would fall into this category... roughly.).
(2) Classic Rap (think Public Enemy, Run DMC)
Ahh... The classic. When squares in business suits try to bust out rhymes, this is the stuff they go for. If you don't know if you're listening to classic rap... Check for a long, high pitch noise, 'breaking words' or scratch tatics. This stuff was good, though it gets boring (becausethe same, short ass beat was repeated over and over again) or the high pitch noise takes you away from the rhymes and more to it's annoyingness. This rap started in the late 80's and ended by 94. Of course, there's sub-categories to this, but I don't wanna get into them. I'm lazy.
(3) Gangsta (think NWA, Dr. Dre, 2pac, BIG)
Now here's something nice. Hardcore, violent and blunt as a candle stick to the back of the head. When people complain about rap being too violent or degorgitory toward women... The examples usually come from here. It's got a lot of sweet sounds and the rhymes was kickin', but damn if it didn't get repedtive with the same theme of hoes and violence. Of course, there's songs that tryed something new.. but.. my minds drawling drawling a blank.
(4) Contemporary Rap
Unless you're Ludacris, DMX or Twista... You probably suck! Expecially you, Snoop Doggy HAS BEEN. (I kid.. There's more rappers that are good, but I can't think of them.) If you want an example, turn on the radio. Boom. 50 Cent is also alright, but everyone wore him out way too quick.
Sidenotes:
[White Rappers] I do think it's possible to have a good one. Too bad 'big business' doesn't. Vanilla Ice is a one-hit lying wonder. Eminem had to crack into the industry by being a jokester and still relies on humor and sillyness (see all of his singles), even though some of his more serious songs are a lot better than the 'joke' songs. ICP isn't rap.. It's a bad joke that won't die.
[Little Jon and the East Side Boyz] WHAT?! Sorry.. bad joke. His voice can get on my nerves, but anyone that can turn 'Crazy Train' into a beat for a rap song needs a hug.
[Bitches, Gangs, Bling, Hoes: the Dark of Rap] What we see too often from Gangsta rap on is this 'ghetto' theme of hardlife and rather rapping about change, must rappers lay out rhymes about being tough, etc. Now, one would instantly think about jumping on the consertive bandwagon and dissing them. Thing is, we're not analyzing the true nature of this phenom.
To simply put it, what sells is being tough. This has been obvious since the 70's. You see this in rock. You see even in country. The same is true for rap. It appels to the needs of the stereotypical man. Of being tough, of being hardcore and of being different. Songs about other things like social awareness don't sell because people are retards and they can't wrap their mind around a subject of that nature.
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In summary: Rap/Hip Hop is good (to me, at least) when done right and there's something it has to say or when I just wanna feel tough. It's still not as good as Punk, though.
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Shame on Us for All We Have Done.
May God Have Mercy on Our Dirty Little Hearts.