Yeh it's the kick-drum that is causing the most bass imbalance there I think. Reduce the bass on that and may be add a bit of dynamics to the bass track itself to ease the boredom of the bass track. May be you could start out with just singular bass notes for each bar/measure, then work it up to that faster bass riff you have created. Other than that, I would have added a little rhythm as well.. whether or not the rhythm section is done with a guitar or not is not important.. just needs something to fill out the mid-range a little more.. possibly some strings (viola and cello) even.

EDIT:

Since I already have a backing track of my own that I have been using to practise training my ear to tell the difference between different beats/grooves, I thought I'd share it. The track starts off with a very light and gentle samba beat, and then later on it becomes a more solid feeling rock beat. The chords are very basic and the structure of the piece is very basic also. It's like this:

60bpm 4/4
// E / E / A / A
/ E / E / A / A
/ E / B / A / A //

Yes.. 60.. it's really slow but like I said in another thread on shredding, playing this slow has really helped me develop more speed than I had before. It can be a really enjoyable jamming loop although the quick transition back to the light samba beat is a bit off-putting. The only reason I am even bothering the upload it is so other people can may be use it to practise with... otherwise it is pretty bad.. hehe.. :)

You can find it here:
http://www.soundclick.com/bands/page_music.cfm?bandID=737179

Hrmmm.. one more edit.. I just listened to the track using SoundClick's player and their player actually plays the song slightly slower than it should be. So.. make sure you download the MP3 instead. I listened to both of them at the same time (i.e. the original mp3 and the uploaded one) and they are quite out of sync. The downloaded one has a much nicer sound to it naturally.