I would disagree that the early Metallica albums are samey. They're quite distinctive in the context of metal, in fact.
KEA was a very NWOBHM/proto-thrash effort. You could easily call it thrash metal and be completely correct, but it was still early days for the whole movement and so there was a higher prevalence of the less extreme NWOBHM elements. In addition, Dave Mustaine had a hand in most of the songs. Many songs nodded in the direction of blues due to the melodic parts of the riffs. Hetfield's voice was quite strained and leans more heavily towards early hardcore punk than anything at this point (although his later vocal efforts would considered much more "metal", they wouldn't largely influence the melodic thrash scene, because everyone figured that he was a dick)
RTL was probably their most extreme album. From this disc, Metallica could have easily launched into more extreme thrash along the lines of later Dark Angel. Just listen to Fight Fire With Fire and Creeping Death. There was a much more diatonic feel to the riffs, and Hetfield had less of an edge, but more variance in his voice. This album ushered in the first instrumental that the whole band would play - Call Of Ktulu. Apparently, Mustaine had a hand in this one as well (as well as Ride The Lightning, officially. My gut feeling tells me he did some work on Creeping Death, too). Irrespective, it was quite a bombastic album closer, and the first album lacked that kind of powerful, but not quite epic mood altogether.
MOP is more melodic in general. This is where Metallica began to stray, as they developed some odd fetish for soft interludes. Okay, it works fairly well in Master Of Puppets, due to the powerful re-entry, but I feel it hurts songs like Orion that are otherwise very good. At this point, they seemed to learn from other extreme metal bands from a melodic standpoint. The melodies here are darker and more exotic, like the little lick that sees the opening of the Master Of Puppets chorus, based on the B phyrgian dominant scale. Flat 2nds and all that. To round it off, Metallica developed their power-tendencies a bit further. All in all, the album picked up some darker melodic tendencies and some compositionally softer ones.
You can always tell a song that's from AJFA because there is no bass. 'Nuff said.
A lot of people understandably hate Metallica because they were in the position to show metal to the world like never before, but instead went on to make Load, ReLoad and St. Anger. In addition, they spent their time ramming drum sticks up groupies' asses and making them lick that shit. Gee, thanks Metallica.
It's kind of nice to get the implication that Kirk is sick of all the bullshit, though.
Not that it matters, though. Pantera's Cowboys From Hell was an impressive entry into the 90s for metal (it's a shame they could only do one album that well, but damn, it kicked ass), and Blind Guardian hooned along with 1995's Imaginations From The Other Side. Plus, there was the whole death metal and black metal explosion. Metallica are still dicks for backstabbing every metal fan ever, though.
(Megadeth was always better, anyway)