Very fun, I'd love to see it extended!
I really enjoyed this game! It's very simple, and although I found it a little tricky to start with (I don't have much experience with tower defense games) it's one of those games where once you understand a few basic principles, you won't lose, and you even start to invent your own challenges. (E.g. get to the end without losing any lives; after I had won once, I played another couple of times to achieve just that.)
Given this, many people would enjoy seeing the game extended a little. For example, one reviewer suggested having units which are immune to darts, which would make it much more challenging in later levels if you hadn't built any bomb towers. It might also be interesting to have a few more intermediate towers; again, as observed previously, the ice/bomb tower combo is expensive and hard to pull off effectively.
Finally, while most reviewers thought the game was easy, a small minority seem to find it a little too difficult. It is tricky to start off with, since a slight miscalculation can make you leak; but in later levels, if you've planned carefully, you don't need to worry so much. The key is to experiment, and work out what rules to follow when placing towers. If you don't have much success, PM me and I'll give you a few pointer (or look back through the reviews).