It's getting a 4, and it easily and I do mean easily could of been a 5 for a multitude of reasons.
Here is a few why it's not getting one.
1. Upgrades, I don't feel like I'm earning these, they are just handed out to you at the end of each level which I have never lost since I started playing it. You collect these little tool boxes for score, but do they really do anything?
What you could do? Make the upgrades purchasable at the end of every level, it means not only do you gotta cover your butt, but you also need to make sure you're getting enough money to get any upgrades you need to advance.
Some are useful, highly in fact, and others...Make me want to cringe by the time I get them.
Example, The fire bomb thing? Lovely special weapon drop.
The slow down time? Another example of pure epic good.
Then we get to the acid? I think it is? Not sure this thing...It's a horrible weapon to have lumped onto you when your main gun is shooting 4+ bullets in multiple directions all over the place and doing a slightly less damage factor.
What to do? Either make the acid shoot like the gun in multiple bursts but at higher power, or make it a HELL of a lot more powerful, because when I get this...It makes me have to worry I will get smacked by a huge wave and then just be boned.
I haven't really been challenged beyond the occasional use of the acid spray in which causes it to get difficult since I can't take out multiple enemies. As you progress you get power, and it's pretty easily gotten to the point where enemies aren't much of a threat.
I don't feel like I'm progressing in a game at all, I feel like I'm doing the same loop over and over and over, like there is no goal for me to reach, I don't know if it has a set number of levels? Or if it's just endless. If it is endless, make that an option don't force that upon us as it makes the game lose luster FAST.
What do do with 3 and 4?
Three is easily solved by making an upgrade for trade in point system, as noted earlier in the post this will not only boost the longevity of the game, but it also can add difficulty as well. Now you maybe thinking how? Well if you don't catch enough and you get to a hard part in the game, you're now at a challenge.
The last point also ties directly into point 4, by making an end game, by breaking it into "levels" the player gets a sense of "Doing Something" Vs Oh look...Same old thing again with no end in sight. A level counter adds a mind set of oh, there IS a goal, what it totally is at the moment I don't know...But I sure as hell want to find out. The last level could feature perhaps a rogue kitty armor that humans have gotten ahold of? And with that a small cut scene of kittys taking over the world. Just some basics, nothing needing to be extremely flashy. With the addition of this you could add an "endless mode" Where you could put up a high score counter, and let people test their lasting skill vs one another. Which further adds to the level of play in which your game can accumulate.
Basically it's got more than enough raw material here to make it to a 5, if not higher rating because it's fun, it pulls you in right away with an odd concept and it keeps you interested in playing it for awhile. The issue is that when you notice you really do feel like you're going no where it starts to lose it's shine, which it doesn't have to with some basic fixes. If you could make those, or perhaps make a sequel to the game, with those changes you'll see a large positive turn over from the masses.