Limitations are 16x16 max build size - no height restriction (irregular base is OK, but should fit within a 16x16 footprint, and foliage overhang is OK), one minifig max, and the focus should be on your tree.
MOCs will be judged on the quality of the tree, presentation, and overall scene composition.
I wasn't going to enter, as I find good trees quite difficult, but I whipped this one up that I am quite pleased with! While I wouldn't like to say it was my 'best' tree (I think I will always be searching for the 'best' Lego tree!), I like it as it is simple to build (see the breakdown pic on Flickr), uses common parts and gives a depth I felt was lacking in some of my previous attempts. It is based on this tree by Brother Steven.