There's nothing wrong with Mighty Arrow. It's your typical Pale Ale--light bodied, hoppy, very low malt flavor, some citrus flavor. You know it. But, like so many other pale ales, Mighty Arrow doesn't do much to this brew that would make it stand out. You figure the least they could do would be to throw in a random extra ingredient, a-la Rogue's Juniper Pale, but nope. New Belgium is content to use the standard no-frills recipe. The result is a standard, no-frills beer.
The failure of Pale Ale brewers to do anything interesting with this brew is all the more surprising when you consider that Pale's slightly edgier cousin, the IPA, is one of the more interesting brews out there. From Ruination to 90 Minute to (see just below) Idiot, the IPA genre has all sorts of interesting and tasty variations. But in the Pale Ale arena, Sierra Nevada continues to reign supreme. A couple other breweries have made beers that are, from a taste standpoint, arguably superior to Sierra (Deschuttes and Anchor come to mind), but these just tinker at the edges with what is, at heart, a very uniform genre.
I really like Sierra--and Mirror and Liberty for that matter. But I have yet to find a pale ale that really rises to the level of a world-class beer. Some might say that this is a problem endemic to the genre. I really hope this isn't true, and I will keep looking. But I can say with confidence that Mighty Arrow doesn't point the way there.