Sometimes you have to double tap multiple times for the player to start sprinting. Also sometimes when you are sprinting you slow down and have to stop and start sprinting again. These things get you killed when you are fighting other players and are very frustrating. This might be a bit hard to reproduce but it does happen frequently, and it's very aggravating.
When you start sprinting you should stay in sprint mode no matter what until you release the forward key. This isn't the case when you run up stairs. Or if you try to jump over a block and catch the top of it slightly, you stop running. Having to keep re-sprinting makes the mechanism feel clunky and cumbersome and is frustrating.