Hah, this is a fantastic way to tie in the optional theme! The feature-set is great too (I especially appreciate the ability to pause the heart-timer). I might actually try writing with this later for NaNoWriMo.
One minor nit is that I had a little bit of trouble getting oriented at first, since the heart was beating but not at all bothered when I stopped typing (eventually I figured out that I should press the play button -- I imagined the timer would start as soon I started typing). A bit of visual feedback on the play/pause state would be helpful there (an icon over the heart? a color-shift? stopping the heart?).
It'd be neat if all the features could be toggled separately, instead of picking between individual modes (so I could write without the backspace key, without seeing the text, and with a time limit, for example).
One last feature request: a 0.5 and 0.25 speed setting.
Thanks for making this!