Understand that going threadless will make changing your stem height more complicated and will require you to buy a threadless headset and stem. Consider the rake of your old fork. You'll want to match this as closely as possible to preserve the handling characteristic of your frame. Consider steerer material. Carbon forks often come with a choice of steel, aluminum or carbon fiber steerer.
Steel is recommended for heavier riders.
6. Consider design. Ask around about the particular characteristics of each fork you consider. There are always tradeoffs with every design. Consider tire clearance. Some carbon forks don't offer much tire clearance, limiting your tire options.