First, I need to share how I ended up here instead of places like reddit, Digg, etc (which were 'things' at my time). I was an intern going to work at a large corporation for the first time, not really knowing much about the tech world or the people surrounding it. Sure, I had a facebook account and occasionally visited some other sites on the web, but for the most part I didn't bother with the news sites, even though I had browsed several (and various blogs).
I eventually ended up discussing technology and other aspects with my mid-30's boss, who realized I didn't have a good source for informed and intellectual conversations about technological topics. Do you know what he recommended? That I start to read Slashdot. And so I hopped on here ~ 5 years ago, and stuck around for reasons I'll get into later.
The important part of all of this is that it was the current community that brought me in. It was my boss recommending a site, not my own searches through the internet, that brought me here. I highly doubt that I'm alone in this regard. So I want to emphasize it once more: new users are brought in by the community.
So, why did I stay? I browsed for a while, reading comments and articles (I even read most of the articles when I started out). But the articles themselves weren't all that interesting, especially because I could find them elsewhere 4 days in advance. It was the comments. Being able to see and read well thought-out and reasoned viewpoints about topics, to learn about entirely new ideas in layman's terms, to have a system which makes discussions easy to follow in addition to promoting the meaningful comments (as opposed to reddit, which over-emphasizes upvotes and turns commenting into a contest). Nowhere else does this, reddit gets lost in upvote wars/groupthink, ars technica's comment system is appalling, CNN and pop-sci are populated by people who pride themselves on ignorance, etc. Slashdot is thee only place where once can come for these types of conversations.
In short, as so many other posters have already summarized, it is the commenting interface, and community around it, that makes Slashdot. As long as that stay intact, Slashdot will be ok.
So, with that background, a comparison of the current and beta sites:
Before I get into the main problem, I have to wonder what happened to noh8rz10's comment in the beta? It's very clearly formatted in classic, yet somehow that formatting got blown away in beta. You might want to look into that.
The major issue is the substantial increase in white-space. For example, in the classic picture above, one can plainly see 5 different comments (complete with sigs!), while the beta barely shows 4 (and no sigs). What this does is 'space out' the conversations and make them harder to follow, because one now has to spend even more time searching for them (scrolling up, down, etc).
The bigger whitespace problem, however, is the horizontal space. Conversations on Slashdot go well into 10's and 20's of replies, meaning that if there isn't enough space they'll become incredibly cramped and hard to read. As an example of a thread that's only 5-6 replies deep:
The beta can literally only fit 2 posts! The current designs gets 5! Cutting out all that space on the right cramps the posts into an almost illegible format, with the benefit of showing absolutely nothing to the side! This isn't even deep into the replies!
This is THE problem because it stands to kill conversation by making the comment threads unreadable. The comments section becomes unwieldy long and cramped, discouraging people from reading or commenting at all. And when that happens, there aren't comments around to attract new users like myself, people go away, and revenue tanks.
To be perfectly honest, the other ascetic changes don't really matter all that much to someone my age. It looks a hell of a lot like you're trying to copy generic web 3.0 design garbage, but they don't impact the function much.
So, in summary, the main problem is the way comments are rendered in the new design. If you find a way to condense the vertical space and expand the horizontal, you can probably get through this ok, but if the comment format doesn't get updated then you'll be seeing community problems, which will leads to decreased visitors and eventually the death of the site.