I imagine the main reasons people ever look at this anime and say it sucks are that:
a) There is some shounen-ai and shoujo-ai (AKA gay and lesbian love), but it really isn't anything major. It just shows up in an episode and people kiss each other. It's not the main focus and becomes a bit of a sub-plot later on.
b) The show frequently timeskips, beginning when the main characters are 10 and then jumping a couple years to when they're 14 and then again jumping later on to when they're 26. Although this does fragment the plot, the characters clearly are developed after the skips.
I think although the plot is fragmented, it has clearly established arcs and the plot progresses at just the right speed, getting through large amounts of plot quickly but not too fast (like SAO is).
