Great inprovement over previous titles
(1) Improved AI: The fact that the AI actually have set schedules that they follow is very cool and a neat addition to this game.
(2) Weapons: Most people say that the weapons in the previous titles are better than these ones, but I disagree. For starters, the ability to upgrade your main weapons is very handy and a great idea. Fortunately, they upgrade level-by-level (sort of) and you never really need an upgrade, that you can't buy yet, for the level you're on. The upgrades obviously make the level easier to complete thus making it more fun to experiment with new methods.
(3) Locations: The uniqueness of the many levels in this installment of the series are very good. No two levels are very similar, and this is a good thing. Each level is fun in its own way.
Overall this game is very good on so may levels and there are very few glitches and/or mistakes with this game.