I agree that it's important that early adopters have an incentive to adopt early, no doubt, but given the current market, what's their incentive towards: saving or spending?

I imagine for a currency to work, the incentive has to be at least tilted a little towards spending. Economically, that's essentially why we have inflation -- the system can't reward people for just hoarding dollars, because that's essentially an economic no-op. If just holding currency was more profitable than investing it, than why would people bother to invest in the first place? I'm not an economic keynesian, but all the same, I don't think it's an accident that hiding a pile of money under your bed isn't going to net you new money in a functional system.

Right now owning a bitcoin is a lot more analogous to owning a stock than it is to owning a dollar. Which is to say it lacks liquidity.

A currency without liquidity isn't really a currency, it's just an asset. The whole point of having currency is the liquidity of it, and that liquidity generally comes from a somewhat stable valuation.