The whistle isn't bad, it just needs to be lowered one octave or so.
Super high sounds like that aren't always a preferred sound in a song. :P
Melody seems okay, just needs less repetition. Including simple additions in the
melody increases the complexity of the whole song, which I believe is a must-try.
But yeah, effects, effects, effects.
I wasn't happy with my creations when I first started making music, so I downloaded
a lot of sounds from freesound. Sounds like a reversed cymbal or explosion can sound
really cool if certain effects were added to them.
Try out new things like adding new instruments and effects.
Maybe experiment with different song structures, like a slow intro or faded intro.
Also, I like that background sound at 0:46. /o/