Are you a beginner? - This is probably the easiest of all the categories to classify yourself in. You've just taken up the sport. You might not even own a rod. Your biggest concern is (or should be) getting out on the water.
What makes a rod great for beginners? - You're just starting out so the most important thing is getting out on the water and having a great time. Rods that are great for beginners are durable and have an awesome warranty (because let's face it, you're probably going to slam a conehead into that blank a few times). They are also inexpensive so that you can start fishing without breaking the bank. Performance isn't as important because not only have you not mastered your cast, but you haven't even decided if your really like fly fishing. Equally important, is a rod that's going to help you improve your casting, and to do that, you need a rod that loads easily and gives you the proper feedback.
Are you an intermediate fly caster? - The intermediate caster is the grey area in between beginners and advanced casters. And it's where most of us fall. You can cast reasonably accurately at all of the distances that really matter (20-50 feet). You fish more than just a few days a year.
What makes a rod great for intermediate casters? - Caught the bug have you? Now that you've decided that fly fishing is a serious hobby, your rod has become a serious tool. You're starting to develop your own casting style, and you need a rod that will help you grow and get the fly to where the fish are. Accuracy is important, but the most important factor a rod is feel. Weight starts becoming a more important concern as you start spending more and more time on the water, and you might be willing to splurge a bit on a rod that looks as good as it casts.
Are you an advanced fly caster? - Can you double haul? Can you *really* double haul? You're an advanced caster when double hauling is second nature and you're able to use that to load your rod and cast further and more accurately than the intermediate caster.
What makes a rod great for advanced casters? - At this point, you can cast any rod that's put into your hand, but not every rod is going to feel great or optimize your time on the water. You know what you're looking for in a rod. You need an action that matches your casting style. You want a rod that's specialized enough to meet your needs. You're not looking for "just another rod". You've got that already. It's not 'a 3-weight' it's an 8'6" 3-weight with a medium action for the small streams that you fish, or a 7-weight with a sink-tip for delivering streamers to big browns. It's not just about getting the fly to the trout, it's about presenting it well.
Are you an expert fly caster? - Bring it on Rajeff. The fly rod is an extension of your hand and you can make that line dance.
What makes a rod great for expert casters? - You already have a quiver of rods and know exactly what you're looking for. Performance matters above all else.