So you wanna write StackOverflow in a weekend.
Let's assume, for sake of argument, that you decide it's okay to write it in ASP.NET MVC, and that I've decided to hand you the StackOverflow source code, page by page, so you can retype it verbatim. We'll also assume you type like me, at a cool 100 WPM (five characters per second[1]), and unlike me, make zero mistakes. StackOverflow's .cs, .sql, .css, .js, and .aspx files come to 96.5k. So merely typing back in source code that already exists will take you five hours if you make zero mistakes and deploy on the same platform they're doing.
Except, of course, you're not doing that. So even assuming that it took you a mere ten times longer to design, type out, and debug your own implementation, that already has you taking more than your whole weekend, even if you stayed up coding straight--and I don't know about you, but I am okay admitting I code considerably slower than ten times slower than I copy existing source code.
Well, okay, I hear you say. Not the whole thing. But I can do most of it.
Okay, so what's most? There's simply asking and responding to questions--that part's easy. Well, except you can't let people upvote their own answers, so you need to block that. And you need to make sure that you don't upvote or downvote a person too many times in a certain amount of time, to prevent spambots. Probably are also going to have to implement a spam filter, too, come to think of it, even in the basic design, and you also need to support user icons, and you're going to have to find a sanitizing HTML library you really trust and that interfaces well with Markdown (provided you do want that awesome editor they have, of course). You'll also need to purchase, design, or find widgets for all the controls, plus you need an administration interface so that moderators can moderate, and you'll need to implement that scaling karma thing so that you give users steadily increasing power to do things as they go.
But if you do all that, you will be done. Except, of course, for the full-text search, espcially the search-as-you-ask feature, and user bios, and having comments, and having a main page that shows you important questions but that bubbles down steadily. Plus you'll want bounties, and handling multiple OpenIDs per user, and having email notifications, and supporting JavaScript-smart tags, and having user-configurable badges (you are going to do user-configurable badges without forcing your user to learn PHP and write hooks, right?), the history of your karma, upvotes, and downvotes. And the whole thing has to scale really well, since it could be slashdotted at any moment.
And we haven't even touched things like supporting upgrades, internationalization, karma caps, CSS so it doesn't look like ass, and all the little things under the surface that no one ever sees, but that you need to run the site.
To be really blunt: anyone in this forum who honestly thinks they can do all that in a weekend is so ridiculously full of himself that I want him very far away from any project that I'm involved with.
[1] http://en.wikipedia.org/wiki/Words_per_minute