However, that can happen whether it grows quickly or slowly.

Yes. It's an issue regardless. But if pursuing a strategy of rapid growth guarantees an influx of negative-net-value-to-LW new users, it's probably a bad thing. (We will all die eventually, but I still prefer there to be fewer mass murders.)

I wonder whether you think this idea would work

I don't have a good enough mental model of the typical first-time visitor to LW to have a strong opinion. What I do think likely is that either it wouldn't work or it would work at the cost of getting rid of that rapid growth you were hoping for. Because I think it's likely that rapid growth by increasing visitor retention implies the sort of change in LW's demographics that I described.

I think it would be a heck of a lot more effective if specific feedback was provided

I have proposed something similar myself. The UI would be tricky to get right. It might well make LW a better place (or it might not; these things have a way of producing unexpected consequences). But I don't see how it would do much to solve the problem I described, if (as I suspect but don't claim to know) it really is a problem.

what if it took, say, three months and X number of posts before you're allowed to vote?

Might work. Has anyone reading this got experience of such systems? (The obvious concern is that it would drive away "good" users as effectively as it would drive away "bad" ones, so that it would slow growth without actually making the overall pattern of growth any more favourable.)

I figured you would all let me know what you wanted and if growth wasn't it, you'd shoot me down.

According to my quick count, 28 different people have commented here [EDITED to add: not counting "metatroll"], of whom 7 seemed definitely in favour of growth (perhaps having thought it through, perhaps just because it's a sort of default goal), one seemed definitely against growth, and 6 seemed definitely skeptical (i.e., saying "growth might not be what we want", basically my position). The other 14 expressed no opinion on the matter that I could detect. Make of that what you will.

Has anyone thought much about what would improve the quality of discussion?

I think a lot of people have, but so far as I know no one (here or elsewhere) has a silver bullet that ensures that a community of unusual people will retain its distinctively valuable characteristics as it grows. (Or for that matter as it doesn't.)