Pretty nice, I'll ignore the venonat resemblence =P
I Think people are reminded of brackenwood so much because not only is the character similar in design, his observance of the world is done very close to the style of brakenwood. (including camera angles, head movement, etc.) His habit of blinking, his walking style, etc all seem like mannerisms that appear over and over again in that series.
Anyway, the reason I'm giving this a 4 instead of a 5 is that the characters themselves were all animated very well, but the backgrounds lack detail sometimes, and the black gloop character seemed out of place.
In terms of background examples, when fluffo was popping out of the snow, you used a gradient on the snowpile which was very...different. I'd have suggested simply getting a brush and shading the pile itself instead of using a gradient.
Other times, the landscapes of snow just seemed underdone, considering half of the movie was fluffo looking around at the scenery.
Finally, my qualm with the black goo monster is this: people associate that kind of design with pollution and evil. It might have not been a pollution monster, but when I saw it the first thing that came to my mind was "Why is there a pollution monster in a place with no humans or polluters?"
Also it attacked for seemingly no reason, which as a "nature" piece (that's what it felt like) he seemed woefully unexplained for the few seconds he was there. Was he territorial? Was fluffo going near some sort of stack of food he was protecting? Or what?
Also it doesn't make sense why it didn't like light. (Or was it just too big for the tunnel?)
I felt that was used in a cliche way and once again could have been solved by a redesign of the gloop enemy's appearance.
He's a monster in a deep underground cave, he'd probably have to have large eyes or other features to make him accustomed to seeing in the underground. (Only for some reason that large cove was very well lit, considering it's in an underground hole while it's storming outside. But that's just yet another nitpick =P) Anyhow, you could've zoomed up on a feature of him (his size, or perhaps large eyes) that was sensitive and displayed it, so when he did see light/whatever happened would make more sense for him to stop pursuit.
Anyway these are stupid nitpicks and this is a really good flash, it's just I didn't even have to think about the movie to notice these things D: So I can't really give it a perfect 5. Still though, I hope you do more as I honestly enjoyed the movie and loved the way you portrayed your character's thought process. =)