For most reserves, it's a pretty easy decision what to put on top once you know what it does when called.
It is? I mean, all official reserves are either nothing, +1 action, or +1 card + 1 action, which just correspond to how strong the call effect is I assume. Not counting Wine Merchant since that's not called for an effect. Is that what you mean though, that it's a simple matter of choosing between those 3? Or do you mean something else; because there's no reason we can't have a reserve that has an actual real effect on top.
Well, conceptionally speaking, Reactions have two parts that are distinct, merged into one card. The Reaction, and the Action. Action-Reserves on the other hand are one thing. Sure, they have that line, but each Reserve ultimately has only one concept. In that respect, Reserve as a type is similar to "attack", where the type describes what kind of thing it is, but doesn't create an additional gap to fill. Reaction on the other hand creates cards with two mechanics, similarly to overpays, on-gains and on-trash abilities. Nothing says a Reserve can't have an interesting top. But you don't need one, either.
What Accatitippi sees as a problem, i see as an advantage: A simple vanilla effect that wasn't cointokenified yet will do for a reserve. For a Reaction, we need not one, but two original ideas that fit on one card. That gives us more to decide on, but it's also a lot more challenging - and i think creating a card as a group is quite a challenge allready. IIt's a lot harder to vote on two different things that are supposed to come together.
Besides, you can easily have a Reserve react to things, too. We don't have a Moat-Reserve, for example. Or one that gets called to island a bought VP card. Or the infamous +buy. Or a Reserve with a cool but hard to trigger calling effect. Or...
Also: Stop ninjaing me while i try to keep up with the other ninjas. This isn't some Shonen Manga, dammit!