No idea why people don't like it.
Other than the brightness and saturation balance (discussed at length http://www.arcengames.com/forums/index.php/topic,10435.msg101979.html#msg101979
and on, and there is even a texture pack posted with desaturated background elements), I love the art. I know a very busy art style may be a bit off putting, but I like it. When wandering through the wilderness IRL, you expect this level of detail and "busyness" in everything, but it isn't jarring or so distracting you can't do what you need to do (or, at least I hope so. A person who finds real-life ugly and jarring would have a hard time living
Two relatively easy to fix criticisms, and one not so easy to fix.
-"boundaries" and "borders" between stuff can seem inconsistent, like some things seem to have a sharply defined boundary or border line, and others don't
-color brightness and saturation balance, especially on background elements
-some of the tiled backgrounds are rather, well, ugly (especially noticeable indoors and in caves)
However, these are mostly "minor-ish" issues in my opinion of the art.