At a glance, Raptor may seem like a typical scrolling shooter. It has all the hallmarks, all the core mechanics. Yet Raptor has a feeling different from any other shooter that I've ever played, because Raptor is fundamentally about attrition.
You can buy various weapons in many scrolling shooters, but it's never been more integral (not even in Tyrian, which is far more forgiving than Raptor). The equipment for sale isn't just minor upgrades or small bonuses; each weapon system makes a major difference in your capabilities and is crucial to your survival. The health system is economy based - if you've kept up with weapon upgrades, you can usually afford to buy enough phase shields to guarantee that careful play with see you through the next stage. However, if you keep taking significant damage then you'll never be able to afford the next step in weaponry. If you can't upgrade your weapons semi-regularly, then your lack of offensive capabilities will slowly result in larger and larger repair bills as the surviving enemies zone you into increasingly damaging situations (and the level designers clearly understood the dynamics of controlling screen space). It's not the instant death, the quick kill, that is the real threat in Raptor; the real terror is the slow slide into an economic pit that will kill even the finest pilot.
Raptor adds a whole layer of risk-reward analysis on top of the traditional scrolling shooter skillset. Each kill means more cash, but going out of your way for kills can result in an even more expensive repair bill. Similarly, it may be better to take some extra hits immediately to avoid getting zoned into an even more damage situation a few seconds later. There is no (significant) regenerating health mechanic in this game, no ablative layer that allows you to take a few "free" risks. Nor are you so fragile that risks are impossible. Instead, Raptor strikes a superb balance that turns every move into a calculated risk - even when you only have fractions of a second in which to decide.
That's not to say that Raptor is brutally difficult, though you can certainly make it so by choice. There are three difficulty levels that significantly alter how unforgiving the game gets. The game is also separated into three "sectors" (each with nine levels) of increasingly difficulty, so starting on a later sector gives you far less time to build up equipment before being thrown into the shark-infested deep end. Starting with Bravo Sector on Rookie would be easy for anyone familiar with the genre, while I'd be surprised if anyone has won a game beginning with the Outer Reaches on Elite. Raptor scales well.
Most shooters are a blend of pattern recognition and memorization, plus the reflexes to respond accordingly. Raptor's economic imperatives take that a step deeper without sacrificing the basic good design of the genre. It's definitely worth the price of admission.