Cropped to a vertical orientation.
Cropped to a horizontal orientation.
I often mention choices about cropping, and in this post I have two versions of the same photo of a jewel beetle, Agrilus cyanescens. The vertically-oriented shot was cropped much more than it’s horizontally-oriented counterpart. They both have their own merits, but this is the kind of thing that gets subjective in a hurry.
Note: I recommend viewing the larger images if possible!
BugPhoto.net 
For the web horizontal means a larger picture, for magazines vertical is the way to go. as most things end up on the web, this theory goes, horizontal wins.
Another problem with vertical, at least on my site, is a semi-unfriendly posting formatting. Artistically, though, sometimes I just prefer the vertical. Web be damned. :)