fairly fast, once I stopped doing a single line at a time (previously I was using setTimeout calls to only do a line at a time, and wondered why it was so slow - it was idle 95% of the time!)
preview, allowing zooming in to interesting areas without waiting for whole thing to render
configurable iteration count, to allow compromise between speed and accuracy - but more importantly allowing the interesting areas to be found quickly and then recalculate at a higher iteration count as needed
ability to specify colour functions for the RGB components. All are functions of i, which is the integer number of iterations completed before bailing out (will be equal to MAX_ITER if within the set itself). The functions should return a value in the range 0-255
a position stack allowing unlimited 'back' operations