The storyline and the graphics were the main strengths and it was that which made me play it to the end. I loved how it had a the reputation and a sense of story progress. And I always enjoy non-linear games with lots of extra places and upgrades however it did get a little bit repetitive near the end with all of the final missions. I did find that the upgrading was a little bit sudden. It was simply evasion and shooting the weaker opponents to suddenly being able to destroy everything. What would also do nicely in a sequel would be some more varying gameplay as it was the simplicity of the game that made the final missions lag a bit.
Basically, in a sequel, it would be perfect with more gameplay features (different guns and gun upgrades) as well as more difficult opponents (with AI).