One thing I like to keep in mind is that most significant proposals fail “on their own” by failing to gather a consensus or to motivate enough people to do the required design and implementation work. It's not really necessary or constructive to “downvote” such proposals when they're just at the stage of one person expressing an idea.
I wrote a longer post about this in this previous thread. Here's the most relevant part:
If a proposal (or a counter-argument to it) demonstrates value to enough people then it will gain support and can be refined into something that can reach consensus. If not, then it won't. Critiques of early-stage proposals in particular should be done with a goal of improving them, better understanding their motivation, or comparing them to alternatives. Comments that are closer to just "voting no" on a proposal aren't really necessary.