As a dog provides a great reason to go for a walk, a fly rod creates the world's best excuse to get out into nature. The main difference between fly fishing and conventional fishing is how you cast; or better put, what you cast. In conventional fishing, the weight of the heavier bait or lure is cast from the rod, pulling the monofilament fishing line behind it. In fly fishing, the line itself is the heavier item, and you cast a loop that pulls a lighter fly (not bait) after it.
Most fly fishing guides have some boilerplate instructions on what to buy and where to go, but they're rarely useful in the long term. In a lot of cases, they're geared toward selling a beginner's outfit, not a specific rod that you'll use for years. This one-size-fits-all approach doesn't take into account your specific application of gear, and you might wind up buying something that's inadequate for the type of fly fishing you actually want to do.
So before you buy a fly rod, you have to ask yourself some questions: What do you want to fish for? What environment do you truly see yourself in? Will you be in small mountain streams fishing for brook trout? Or in large western trout rivers like Montana's Snake River? Maybe you're more interested in pursuing bass on local warm-water ponds and lakes, or saltwater gamefish. Each of these environments requires a different rod, reel, and line.