First of all, Slashdot is commercial in fact if not in spirit. Plastic.com is different because it'll be run by media professionals rather than Taco (thus I don't expect hooters links on the front page of plastic.com), and it's created with a commercial goal apparently in mind.
But what exactly is this commercial goal? It's merely getting pageviews for plastic and partner sites, not exactly an exotic (or commercially successful) business model. From the MSNBC article:
Plastic will draw upon the million unique monthly users of Suck and Feed to start, and hopes to draw
upon another 4 million to 5 million per month from its editorial partners.
Ok. So they're hoping to get a few million people to look at their site. I suppose this is the sort of numbers you'd need to get a worthwhile revenue stream from banner ads. They seem to be aiming to be a general interest /., so getting a few times /.'s visitors might not seem unreasonable.
But a few problems arise when a community gets to that huge a size.
If plastic.com becomes really big (a big if), there is absolutely nothing preventing similar slash or scoop based sites from springing up. It's hard and expensive to produce lots of high-quality content on a daily basis (witness the unprofitability of the most ambitious offerings, Salon and Slate). But it's really easy and cheap (by comparison) to link to such content in slashdot style. And especially when you don't have to code it yourself. I can see no advantage plastic's partners lend to it that would protect it from copycat sites.
Further, after they get a few thousand users, the copycats would probably be better, due to not having millions of participants.
Anyone who's been to slashdot knows what happens when a community gets that big. Even ignoring trolls, you end up in a sitution where people post so fast that your comments are buried unless you post within a half-hour of the story appearing. This promotes short, uninteresting comments and seriously hampers discussion. The most successful "communities," whether newsgroups or mailing lists or bulletin boards or weblogs or whatnot, seem to be moderate-sized for this reason (and others?).
In other words, I think community (I hate the term, incidentally) sites have a cap on the size they can be before self-destructing--no matter how broad their appeal.
Looking at Plastic.com, I also suspect that their focus will prove too broad and diffuse in the long run. Active communities, good or bad, are somewhat focused. k5 is one of the least narrow I've seen, and it's far from being for Joe Q. Websurfer.
(The site might even be harmed by its close association with Wired and the rest. If they post stories mostly from their partners, and I'm sure there will be a tendency to do that, it'll make the site less interesting.)
But, as they point out in the MSNBC article, it doesn't require a lot of staff to run plastic.com. (Just like Feed produces vastly less content than Salon, and can operate in the black, unlike Salon, which has no prospects of profitability.) I'll be interested to see if it ever reaches critical mass. If it does, it might even be worthwhile. But I can't see it (or any other discussion site) raking in the cash.