Aggron in OU? Yeah I know, it's trite saying that he cannot be OU, but think about it. Remember when Relicanth got Head Smash in Platinum and there were so much hype about it? Now he's listed as NU in Smogon which goes to show that you can have 225 base power moves but still fail if you have bad stats and typing.
Relicanth actually works fine in OU too, but that's beside the point (he actually got Head Smash in DP btw). Relicanth isn't used very much in OU because he has weaknesses to common attacking types while only really having a resist to Fire, Ice, and Flying. This means that his amount of resists isn't compensating for his amount of weaknesses. Also, he's not immune to Toxic Spikes like Aggron, or resistant to Stealth Rock. Aggron also has a million resistances to come in on and set up with.
Also, there was next to no hype for Relicanth as far as I remember.
Relicanth and Aggron don't differ much in stats and movepool BTW. Aggron has 20 BP more attack, but that doesn't matter much considering that it's that 225 move power that's factored in more into the calculations, and the 20 more BP Aggron has only make a slight difference. Plus, Relicanth has a better typing (4x weak to grass as opposed to 4x weak to fighting and ground), better HP (with the same SpD, thus taking special hits better), and 5 point better speed (which actually makes a huge difference). And Relicanth also knows EQ, Aqua Tail (but STABed, thus better), and Rock Polish.
I don't think you've run their attack through a damage calculator. If you had, you'd see that the 20 high base Attack for Aggron makes a ridiculous difference. For instace, on an imaginary pokemon with 300 HP and 300 Defense, let's look at damage with the CB sets:
Relicanth:
459 Atk vs 300 Def & 300 HP (150 Base Power): 246 - 291 (82.00% - 97.00%)
Aggron:
525 Atk vs 300 Def & 300 HP (150 Base Power): 282 - 333 (94.00% - 111.00%)
When a pokemon is relying an extremely high BP to do good damage, each Attack point makes a huge difference, which is why there's such a large difference from only 20 base Attack.
Aggron knows Earthquake too, so I don't know why you mentioned that.
Also, when you look at a typing, you seriously shouldn't just look at weaknesses, because that's only seeing half the picture. Resistances are extremely important, especially for something like Aggron, which only needs 1 turn to come in on a resist to severely hurt something, or to setup a sweep. In addition, Aggron can compensate for the Water weakness with SS support, and Magnet Rise is rather popular, so even though his typing suggests it, his weaknesses aren't quite as easy to exploit as they look.
So yeah, I don't see how this situation of "Aggron is OU" any different than in Platinum when Relicanth was hyped because of getting Head Smash.
Hell, I wish Relicanth was OU, since he's great there, but as it stands, people are too scared to use him because his typing offers like no resistances (though it's one of the best offensive combos in the game), PS. his special defense is 5 higher than Aggron's :]