i'd like to weigh in, but i only got a few minutes, so i'll try to be as concise as possible.
i believe that to judge a player u need to consider his entire carreer (which is why i have such a hard time judging youngsters.... their carreer is still non-existant). even great players can have one or 2 under par seasons, so the wider the time-span u consider, the more accurate is your judgement.
and in my personal opinion (which probably isn't worth much, as i guess i haven't followed kompany as closely as gerd and godotelli), kompany has been, for most of his carreer, an absolutely fantastic defender.
i also believe that, in order to tag a player as "top class", u need to compare him to his current peers only. if u're trying to establish if a player is "world class" u should only compare him to the best players in that position
right now.
if u try to compare kompany to the likes of nesta and baresi, then u're not just trying to establish whether he's world class anymore... u're trying to figure out if he's one of the "all time greatests" in that position (which is very different from being top class).
so does kompany compares to the likes of baresi, maldini, nesta, beckembauer, scirea? of course not. but then again there's no player who is even close to that level today.
does he belong to the same class of thiago silva, hummels and the others best defenders out there today? in my opinion he does (but that's just me).
one final point. chances are we will never see another baresi or another nesta. and not just because the officiating style of the refs shifted the balance in favour of offensive players (although that's a very good point ryan

).
the truth is we don't teach defending anymore. the advent of zonal marking marked a cultural change in how we approach defending. defending today doesn't depend on individual technical skills as much as it depends on the coordination between the players who form the line.
trainers and academy coaches soon adapted to the change, and, as a result, today we have a new breed of central defenders. players who, as they were young kids, spent most ot their time in the academy, learning how to read an offence, how to coordinate their movement between each others, how to shift the line back and forth, how to swap their marking assignements without changing their actual position.....
that's all great. problem is they focused so much on theese aspects of the game, that they didn't really have any time to learn how to tackle and how to mark.
there are some aspects of defending which are completely overlooked by modern trainers and youth coaches. and i'm not saying this coz i have some specific insight or coz i have some friends who work in the business..... i simply see it every time a watch a game in tv.
modern defenders have an amazing vision and a spectacular timing and coordination, but their individual technique is simply non-existant. even the best of them often make amateurish mistakes, when it comes to certain situations.
u can see it for yourselves. this weekend focus your attention on defenders (especially during those close-ups they often show on tv).
look at their feet-placement. they always keep their feet slightly apart (which is correct) but they never keep their feet pointing in different directions (kinda like this: \ / ) which is how a defender should always keep his feet while marking (coz it allows quicker lateral position adjustments and it decreases the gap between your feet).
they always keep their eyes on the ball and on the opponents feet and never on the opponents knees and calves (which is what they should be looking).
when they're trying to figure out where the ball carrier is going to go with his dribble (left or right) they always look at their eyes and shoulders (which is why theyre so easily fooled)... and, again, they never look where they should be looking (knees and calves).
they often keep a low center of gravity (by bending their knees), but they never even try to keep a straight back (which is just as important as lowering your centre of gravity).
when they're performing a front-block tackle, they usually lead with their preferred foot (which is just a ridiculous amateur mistake).
now i'm not saying trainers and academy coaches don't teach the basics of defending anymore (that would be ridiculous).... what i'm saying is that they simply gloss over the rudiments of individual defending and spend most of their time focusing on other aspects of defending (mostly line movements). simply put, it's tactical teaching over technical teaching.
but theese aren't aspects of the game u can simply gloss over. feet placement is to defending what guard position is to boxing; as u're a kid u're supposed to spend days... weeks focusing on how to keep your feet when approaching the ball carrier, or how to keep your knees and back.... or focusing on keeping your eyes on those parts of the ball carrier's body which can be good tell-tale signs of what he is gonna do next. only through endless practice theese habits will become natural (u have to repeat some movements so many times that they become second nature.... so u don't even have to think about "how to keep a proper stance" anymore).
and anyone can see modern day defenders (even the top ones) never really acquired such familiarity with the most basic rudiments of marking and tackling.
zonal marking changed defending for good. and there's no doubt defending today is a much more complex and collective effort than it was 25 years ago.... the only downside is that we're so focused on teaching our kids the dynamics of a defensive line, that we don't spend enough time trying to teach them how to mark and tackle. and as a result, body-check is all there is to marking today.
and that's the difference between thiago silva, hummels, chiellini, terry, even ferdinand and pique...... and the likes of nesta, baresi, wierchowood, passarella, maldini, blanc, thuram, scirea, kohler, sammer.
those guys didn't just have the talent..... they also had the best training and education a defender could get. and that training and education allowed them to hone their skills to a level which is simply unreachable for modern defenders.
so much for conciseness, huh!
