Might I suggest http://scicomp.stackexchange.com ?
While the StackExchange system isn't the best - and indeed isn't designed - for "discussion", I've found that many "What did you think of this paper" type questions can be phrased in SE-compatible formats. CrossValidated has a semi-periodic "Journal Club" bit, and questions and musings about scientific papers come up a fair amount there.
I think if framed correctly, they might find a useful home here.
Generally though, I think the online discussion of scientific papers suffers from a few problems. Generally, the two I find the most problematic:
- Lack of a clear community to talk about papers in. Essentially, the problem your question is looking for an answer to. I haven't found a really good general purpose one, though I would love to if I did find it. There's blogs and the like, but even the ones talking about peer-reviewed papers are somewhat one sided in terms of their communication, and not great for anything but transient chatter.
- A hesitation to talk about that online. Among colleagues, it seems somewhat easier to summarize things like "Bad paper is bad", or slice apart someone's methodology. I'd be somewhat more hesitant to do that anywhere where my identity is both traceable and the conversation is saved for eternity (the internet).