Pro: The concept is good, and there's a lot of room for growth.
Con: As it is, the controls are slippery, the enemy movements predictable, the graphics meh, and the gameplay gets boring quickly.
Overall: Some interesting ideas that could really go somewhere, if the overall play experience is improved.
Suggestions: I really like the kami spirit aspect to the story. Reminds me of Okami, and I think that's what you should aim for with the graphics. You don't need to achieve that level of graphic artistry; that's really too much to expect for any browser-based game. Just keep that idea in mind if you decide to revamp this game. Otherwise, I'd work on setting up a dynamic method for determining where enemies enter the field, and go with the standard system of waves or levels, increasing the number, and difficulty, of enemies by level. Oh, and you can't go wrong with upgrades for this type of game. Everybody loves upgrades. I think you could also build on the 2 player aspect of the game by making it two characters played by one player. Split the game into two screens, with one character protecting the trunk and the other the branches.
Tips for players: summon Player 2 immediately and place Player 2 by the trunk of the tree, facing left. Leave him there. As long as his shots go past the edge of the tree, he will kill the walking spirits before they reach the tree. Then hop around as Player 1, holding down space to fire continuously, and kill all the flying spirits before they get to the tree. With this strategy, you are nigh-invincible. You'll have to mess up really badly in order to die. You'll probably get bored long before that happens.