hmm... now both techniques you describes works. It might have been a focus issue from the browser.
I played a little longer and got to a place, that was really dark (running to the right), it felt a little random just running to the side. Maybe some more interaction and story can be introduced earlier. Then it wont feel as, I just run to the right, because that is what a normal platformer does.
Generally, the amount of time you can wait between important things is somewhat correlated to how much a player has played. So the first few minutes, should generally have a lot of things happening to pull the player from what ever they are doing and into the world. This normally cannot happen through the entire game, soother things needs to take over (here the actual game mechanics takes over).
I struggle a lot with getting feedback from others, have you tried watching somebody play the game? Without giving them to much intro, Just hey play this game and let me look at you (even though it sounds a little creepy :) )
Good luck with everything =)