Take, for example, the different planes. It's not simply a case of just looking different, but they fly differently as well and you have to choose the right plane for the right mission. Where a bombing run might be a breeze in a Hawker Typhoon, you'd struggle in a Supermarine Spitfire. Another area that works well is the powerups. Collect ten of these and you'll upgrade all your weapons, and, just like the different planes, the upgrades are really noticeable as well.
Machine guns make a bigger, chunkier kind of dakka, and floating mines become absolutely lethal.
The upgrades themselves work very well, but this reviewer can't help but think the developers missed a good opportunity. If only you could keep your upgrades from mission to mission that would add immensely to the enjoyment and at the very least would help to keep the game flowing, which at times it has difficulty doing. The only reward you get for completing a mission is a medal you don't see unless you jump to the pilot roster and occasionally a new aircraft, which isn't always better than the ones you've already got, and in some cases can have exactly the same statistics. Instead, you're thrown back into the missions with nary a whisper of congratulations. Gameplay is also short-lived.