This is an answer in three parts.
Part the First: Recommendation Questions And The Stack
Your question falls into the category known as "shopping questions," which most Stack sites have categorically given up on after years of poor experience getting them to fit the Stack's format.
RPG.SE, however, doesn't want to issue blanket bans for something which we see as a crucial part of our particular bailiwick. Subjective questions have stricter criteria across the whole family of sites and to keep recommendation questions workable in the Stack context RPG.SE has assembled some specific guidelines for them.
I probably should have tagged your question with game-recommendation and published-adventures when I first saw it, with the first tag's attendant guideline links, but I was in a hurry and just slapped out a quick comment. We used to have to go around to each recommendation question and manually add comments like this one:
but now we have this alert box:
I'm gonna go add those tags after I'm done with this post.
Part the Second: Teasing Out Detail and Avoiding Open-Ended Criteria
So, let's look at your question's criteria and talk about it in the context of RPG.SE's recommendation guidelines. You give a lot of really useful context (edition, level, the kind of adventures it's setting up for), but then you undercut that by saying you don't actually need answers which act on that context. You're asking for:
An adventure in any D&D edition, published by anyone, which effectively introduces the Underdark --or maybe just take place underground-- and which the answerer liked. And maybe it should have a dragon endboss? I'm unclear on that point, it seems like an aside.
Well, the "remember fondly" bit is the easiest to talk about so I'll hit it first: it's a criterion for which every answer will be equally valid. "What's your favourite X?" questions are specifically called out as bad for the site, so we'll strike that out and trust our citizens not to recommend junk. (I really hope most recommendations aren't for things the answerer didn't enjoy!)
So we're left with "every adventure that introduces the Underdark and maybe has a dragon" and nothing to differentiate which you'll find more useful beyond that. Although you say it doesn't necessarily have to introduce the Underdark, just take place underground? I don't see how answers to this can be anything except a bunch of random guesses. Voting will break down along popularity lines rather than usefulness and the Stack's goal of "neatly sorted pile of questions and answers" will fail.
Let's look again. One common rule of thumb is not to leave things too open-ended unless they have to be. It's really tempting to widen our criteria so that we get more answers, or our questions will be useful to more people, but our answerers enjoy challenging the frame of the question enough that there's no need to invite us to do it. Answers to specific problems are more useful than vague generalised suggestions. With that in mind:
- You're playing in D&D 4e. We'll stick to asking for 4e adventures from WotC. Underdark and Drow lore are drastically different depending on the edition, so this reduces the chances that a recommendation will be useless because of lore drift.
- You're focusing on the Drow later on, so rather than a general introduction to the Underdark let's ask for an adventure which gives the players a primer on Drow abilities, culture, and lore. That'll necessarily include some other Underdark stuff too, I'd hope.
- You're running it at low Paragon tier. Why not include that as a criterion too, instead of an aside?
Now you've got this:
A 4e adventure for a low-Paragon party which eases them into the Underdark, introduces them to the Drow, and has a dragon for them to fight.
Woo! This I'd be comfortable going to press with, no problem. It's got narrow, clear criteria which let us identify a "best" answer. If such an adventure doesn't exist within your precise requirements, our citizenry will probably recommend the closest possible thing.
However, I wouldn't dream of editing your question to reflect these changes because I made several leaps and extrapolations which may be incorrect. Since you've seen my thought process now, hopefully that'll help you develop more accurate criteria in the same vein.
Part the Third: On Closing Quickly
As for being trigger-happy on closing, well, one of the big reasons to close a question is to protect it from accruing answers that aren't actually useful. That kind of answer wastes the answerer's time in writing it, wastes the community's time in moderating it, and clutters things up betweentimes. This is only effective if we close questions quickly. Once improved, questions get re-opened just as fast as they're closed--often faster!
On the other hand, leaving questions open on virtue of what they might become (rather than what they are at the time of voting) is helpful to neither the querent (who is often unlikely to improve his question if he can still get answers to it, however poor they may be) nor to the site.