The thing with the trees was cartoonish; they needed to think about that one a bit more. The orcs and some of the monsters look almost believable, the Golom character was fabulous, but some of the things you see, such as the Belrog, still look cartoonish and the movie seems too busy at times, i.e. too many weirdies flying around.
Moreover, the plot and basic story do not really seem altogether plausible. By way of contrast, the thing which really jumps out at you watching Reservoir Dogs or a couple of Quintan Tarrantino's films is the way in which the story strikes you as SO plausible that you walk out wondering what keeps stuff like that from happening more often and, in that sense, Tarrentino's villians are a whole lot more believable than Tolkein's. I mean, Marcellus Wallace just has it all over Sauron and Sauroman.
Aside from that, the Hollywood lefties basically hate Quintan Tarrantino to pieces, and the idea of Pulp Fiction being named one of the hundred best films of the 20'th century totally fried their minds as I hear it.