It feels disappointing that you kept mana open for something that cannot be used as a combat trick - okay you advance through your curve if successful, but you waste a turn's worth of mana if your opponent simply doesn't attack.

Risk/reward on this keyword mechanic as a whole is not enticing enough IMO.

I didn't consider how poorly this mechanic would likely be against control decks.

Do you think the mechanic could be salvaged if expanded to sorcery spells as well? Alternatively, what if reserve was triggered by creature blocking and not exclusively your own, so that a player could cast a creature for its reserve cost if an opponent's creature blocked that turn?

It unfortunately doesn't solve the initial feel-bad problem. If you save this creature to get a surprise creature when your opponent attacks, and they don't attack, then you wasted a turn, and you feel dumb.

Personally, I think reverse Reserve is a better ability. You can cast this creature for cheaper only if your opponent blocks. That also encourages mind games when you attack with something small. Do you have a trick? A Reserve creature? Or is this just a bluff? It also encourages attacking, which is always good.

With the way it is now, Reserve disinsentivizes attacking. If I had a 1/1, and you had a 0/1, and I knew you might have this in your deck, I probably wouldn't take the risk... which leads to some very unfun, non-interactive games where we just stare at each other.

There's no real 'gotcha' moment when Reserve, as it is now, does work. Attacking is something players should always be doing when they can. You never get to feel clever when your opponent attacks with his 4/6 into your 1/1. That's what they're supposed to do.

I guess a strictly reverse Reserve would be simple.

Is feeling "clever" really that important? The concept feels a self-congratulatory to me.

So this can be cast at instant speed for its reserve cost? IMO that isn't clear enough and I might have guessed otherwise. I would add "as though it had flash" in there somewhere.