I'm really glad you posted that Wanderer, because it made me realize that your perception of weapons in the game is very similar to the way Chris sees things. Chris also compares the game very heavily to an FPS, and during the beta the game resembled an FPS even more. So your thinking is probably right in line with Chris's. You also made me consider why it is the platformers and FPSs are different when it comes to weapons. And here's what I think:
Most FPSs don't have THAT many weapons. Sure, they have some different weapons, but not nearly as many as the number of spells we have in AVWW. So it's not a huge waste of resources if the player only sticks to one weapon. Nevertheless, single player campaigns in FPS games try to get the player to vary his weapons as much as they can. This is why they slowly feed new weapons to the player rather than giving him access to all the weapons at once. First you get the crowbar/knife, then the pistol, then the shotgun etc. You get to try each one out and it adds variety to the gameplay. Then, when they think you've had the most powerful weapons for too long of a time so you're probably not using all of your weapons, they strip you of your weapons completely! Remember that trick from Half Life and a thousand other games? The whole point there is that you start using your more basic weapons again.
There's another important point about FPSs and that's the fact that you have ammo that almost always varies by gun type (Deus Ex 2 being the notable exception). This ammo count limits how much you can use any particular weapon. It forces you to switch to different weapons to conserve ammo. So weapon variation is built into the basic mechanics of FPS games. When you run out of ammo for your machine gun, you have to switch to a gun or maybe even a knife, and the dynamic of the game changes.
But what about multiplayer FPS modes? Other than ammo mechanics (which are baked into the very fabric of FPSs), you don't really see too much enforcement of variation there. For one thing, I think the assumption is that players have already tried all the weapons. More important, I think there's another really big difference between an FPS and a side-scrolling platformer here that makes all the difference. In an FPS, you work in 3 dimensions. As such, the opportunity for different tactics, especially in PvP mode, is insane. Because there is no guarantee of a line of sight to your enemy, you can have ambushes, and you can approach the enemy from above unseen, or from a trench below, or from behind, or flush him out, or flank him. Expert FPS gamers can develop many customized tactics, some of which rely on specific weapons. Thus, the variation in gameplay comes from competing against the ingenuity of other human players because the space of possibilities for different tactics is just enormous. You don't need the variation in weapons to force you to use different tactics. You could have the same weapon and constantly try different tactics, many of which could be entirely psychological. Then you see another player using a slightly different weapon with completely different tactics and you try to emulate that. But weapons are a small part of the picture here. Aside from explosive types, it's mostly just a function of point and shoot. It's really human ingenuity applied to the massive solution space that makes multiplayer battles refreshing.
In a 2d game, this space is reduced tremendously. You see your enemy, he sees you, and you need to hit him. You can jump above him, approach him from different angles etc, but a huge amount of the variability in your tactics depends simply on what you can fire. This is particularly true when you have spells and elemental resistances involved. Some people may have better reflexes, some may have tricks and 'hacks' to deal with certain situations, but ultimately the variability isn't huge between players of similar skill levels. The variability comes from things like your weapon types, the environment you have to traverse, and the enemy types.
In addition, AVWW doesn't even have the single-player FPS mechanics of ammo or gradually introduced weapons to drive people to use different spells. But it does have a whole bunch of spells, most of which are not touched by many people. And over the course of a long game of AVWW, sticking with the same spells is going to make the game boring.
BTW I agree with you that forcing a switch every tier is probably too much. Per continent sounds much more reasonable to me.
And Binding of Isaac really is brilliant. I suggest you give it a try and see if you like it.
Martyn, I really like your idea of varying cost! But what if the game encouraged you to diversify, perhaps modified both randomly and by what you previously picked? I'd reverse your idea. Suppose the game randomly increased the cost of some spells and cheapened others per continent. This makes sense since each continent has its own 'spell economy'. But then, the game would make the cost of upgrading your current spell higher than the cost of getting any other spell. So you could choose to stick with only your current spells and work a little harder to do so, or you could try different spells and save resources. And instead of having to upgrade other spells through tier 1, 2, 3 to your current tier, you'd just buy them at your current tier. I think I saw you make that suggestion on another thread, Martyn.