It would be great if [Enter]/[Space] would clear a wrongly typed word, maybe with some visual indication that it wasn't typed correctly and maybe also show the mistyped word. Having to delete the word because of a single mistake really takes you out of the flow, and the timer doesn't reset till you delete and retype the word correctly.
Displaying the target ms/character for the specified word count might also be beneficial to quantify what you're working toward.