I would extend the model a bit and divide the Gamechangers into two categories: one (and only one) Hook and 1-3 Painkillers. Your Hook is the feature that your market didn't know was possible, but thinks is awesome now that it exists. Your Painkillers fix long-annoying problems in the problem space. If you think of an individual's psychology when they encounter your product, the Hook is what catches their attention, the Painkillers feed into their confirmation bias to make them absolutely sure they want to purchase, and then the lack of Showstoppers prevent any monkey wrenches from jamming the sales channel.
Some examples from famous consumer web projects:
Google Search: Hook = pagerank. Painkillers = clean, uncluttered UI; fast loading
GMail: Hook = 1 gig of email space! Painkillers = search over email, threaded conversations
Google Maps: Hook = satellite imagery! Painkillers = Smooth AJAX UI, local search results directly on the map.
YouTube: Hook = easy upload of all formats, even from cell phone video. Painkillers = in-page embedding, Related Videos.
DropBox: Hook = Just drag to a folder to share! Painkillers = easy web viewing
FriendFeed: Hook = real-time updates. Painkillers = centralizes all your major social networks, supports in-depth commenting.
Apple: Hook = Integrated beauty. Painkillers = good device performance, high DPI screens, long battery life.
One easy way to categorize features into Hook, Painkillers, Showstoppers, and Distractions is to put yourself into the position of the user and try to accomplish the task they want to accomplish. Distractions are anything you grumble about but then go about your business. Showstoppers are anything that you drop what you're doing to fix, because you can't accomplish what you want to otherwise. Painkillers are things that make you so depressed about your task that you think about doing another task entirely. The Hook is interesting, because by definition it's outside of conscious awareness and requires an outsider's perspective to see. But if your cofounder is doing the same exercise, putting himself in the mindset of a user, then you can use that to illuminate it. The Hook is the feature that convinced your cofounder that this was a company worth starting.
An interesting thing is that though the bulk of your engineering time will be spent on eliminating showstoppers, basically all of your marketing should be spent on the Hook. Let customers discover the Painkillers for themselves; if you try to market yourself as solving more than one problem, your customers will get confused and you won't solve any.
Another interesting corollary: this may be why big companies are seemingly incapable of pushing out great innovative products unless they have an experienced product person with wide discretion at the helm of the division. When you market to executives, they will - of necessity - focus on the Showstoppers. If your attention is focused on their attention (which is the usual formula for advancement in a big company), then you will focus on the Showstoppers too and develop a "feature mentality", where you just try and list out features as a way of distinguishing yourself to your VP instead of to users. You'd think the way to solve this is for the executive to focus on the Hook instead, but when they do this, the Showstoppers tend not to get fixed, because their underlings naturally tend to focus their attention on the VP's attention.