Just from the video, it seems that you take your sweet time bringing your leg back down. Now, for practicing for power, that should be fine, but you should also be practicing for speed - try to do progressively more front-rounds as quickly as possible on that heavy bag you said you had. Remember to bring your right hip over more and "swivel" on your lead foot.
Other than that, I'd say your technique is getting there, but you also need to relax more, as Zyph mentioned. By tightening your whole body like that you're just adding resistance to the speed of your kick.
Power comes from momentum, which is not just the force of your kick, it's the mass of your leg multiplied by its speed. Since the mass of you leg isn't going to change much, you should also be practicing for speed, which will give you a harder kick that is more difficult to block.
Hope that helped, "Coach."
(Here's some vids to illustrate what I mean.
Front Roundhouse instruction
Art of Thai Kicking