More physical buttons. Longer battery life. Displays that are clear in direct sunlight. ANT+ sensor connectivity. Data syncing and extended display with bike computers. More different sports, including multisport activities with transitions. Complex structured workouts. Multiple types of configurable alerts. Pacing targets based on a course elevation profile. Daily suggested workouts. Detailed online analytics. The list goes on and on.

If you count the fitness trackers worn by the top finishers in any sort of endurance sports race you won't see many Apple watches. There's a reason for that.