Important considerations when choosing a fly reel:
It is likely that the purchase of a fly reel will likely be one of the more important purchases you make related to fly fishing. Next to a fly rod, it may also be one of the more expensive ones. Fly reels range in price from under $50 to nearly $1000. If you will be fishing small streams or other places where you know the fish are relatively small, your decision on the proper reel for you becomes much easier. Why? Because you will not need a substantial drag system and you will not need a lot of line/backing.
I want to emphasize that matching the fly reel to the fly rod is of prime importance. If for instance you have a 5 wt. fly rod, you will want to get a reel that is for a 5 wt. rod. When you purchase a fly reel, it will specify the "line weight" within 1-2 line sizes (for example, a 5-6 line) for a particular reel. Remember, the fly line size (wt.) also needs to closely match the weight of your fly rod. When a fly reel specification notes the size of the reel, there are things that are a given for that reel size. The reel size takes into consideration not only size of the fly line but also includes the backing (typically braided dacron or other synthetic material) that is typical for that size reel.
I think you need to ask yourself the following questions:
What species will you be fishing for and how big/heavy are they? This relates to the need for a drag system and perhaps the need for more or heavier backing.
What size reel is best suited for my fishing situation? As stated earlier, the reel should closely match the fly rod weight and be able to hold the proper amount of fly line and backing.
Right or left hand retrieve? If you cast right-handed, you may well want a left-hand retrieve - most common. Or, like me, you may choose a right-hand retrieve even though you cast right-handed. When I hook a fish or simply want to wind in line I switch the rod to my left hand.
Weight? Here I mean the actual weight in oz. There can be a considerable difference between manufacturers and even between models from the same manufacturer. Over a long day, that difference could be a factor. Generally, CNC machined reels are lighter than die cast reels.