I don't know...I guess one of the things that amazes me about anime is their ability to maintain good proportions of the human characters and of all other things, in general (in most animes, that is...not all of them).
When I was younger, I used to draw comic book characters all the time, and I must admit that proportionality was always the biggest challenge. Even though I've gotten better at it over the years, I still recognize it as quite a challenge (actually, I think it is the toughest part of drawing, altogether). And I can only imagine how challenging it would be to animate a figure, and maintain those good proportions (once you have them done well in a single drawing) over hundreds and hundreds of frames. The fact that anime accomplishes this...just amazing.
But...when I watch American animations, it seems that I find flaws in proportions all over the place. Take for example, Teenage Mutant Ninja Turtles. I think everyone would agree that the first 5 or so episodes of TMNT are by far the best, as far as quality of artwork is concerned. But even these first few episodes are riddled with disproportionate body parts on all of the characters (i.e., head is too big, eyes are out of place, torso is too big, arms seem to change size when they move, misshapen body parts in general , etc). And as far as the rest of the episodes of TMNT...don't get me started! I don't think they were even trying to begin to think about the possibility of maybe keeping decent proportionality in the rest of the episodes.
TMNT is just one example, but it is by no means the only one that does this. In actuality, it seems that nearly all American animations that have human characters that are supposed to be drawn somewhat realistically screw it up all the time. Yes, I know that TMNT can hardly be considered "realistic"...but body proportion-wise...they (the turtles) were supposed to be very similar to humans. Actually, it is not only limited to human body-like proportions, but to all proportions. Take, for example a sword coming towards the camera. Keeping it proportionate with respect to depth is also a major challenge; one that anime often passes with flying colors.
Anywho...that, coupled with the thick lines that often seem to get thicker, or get thinner with each passing frame...yeah, I know, I may be knit-picking...but, hey...that's my personal tastes for ya.