Warblers aren't all that difficult to ID in the Spring. Their markings tend to be clear and distinct, and though some are a little tricky, most of them are easy with practice. But in the fall, you're dealing with juveniles, fall plumage, and lots of other confusing markers. This bird in particular gave me a real headache figuring out. None of its markings quite seemed to match what I was looking at in the books.
As it turns out, it's a Pine Warbler. It's also the first one I've ever photographed, which is kind of cool. In Spring, these birds are much more clearly defined; sort of like a goldfinch with stronger wingbars and shaped more like a warbler.