Separately, one thing I'd be excited about is filtering by users at companies in a certain grouping (e.g. by Geo, industry, or some other affiliation like YC or 500 Startups alums). Siftery can do this since users have verified business e-mails.
Slicing and dicing in that way by geo, industry and whatnot, would be very interesting as someone looking to build and sell software. It's always really helpful to know what different kinds of prospective buyers are currently using - and if they're excited about it or not.
As a prospective software buyer, however, it doesn't necessarily sound all that interesting. I mean, so what if a lot of companies that are in a similar location and industry to mine use Bitbucket. That doesn't tell me if it's actually great or if they just don't know any better. And if fin-tech companies that went through YC love using Aha for product roadmapping, it's kind of the same thing - what does that really mean?
That said, I can see some situations where it would be interesting. If I'm looking for software that has intense compliance or performance requirements, then know what other people in my industry use would be helpful - so I would know that I'm making a reasonable decision.
Also, what would be super interesting would be knowing what software transitions/replacements people have made. For example, if I'm using JIRA and not really loving it it would be really cool to know what people who have left JIRA have used to replace it. A lot of times I've found myself not entirely satisfied with some work/productivity app and wondered if there's a better option out there, but then found it can take a lot of research and experimentation to answer that question so I just go back to the status quo. Seeing that a lot of people have happily made a transition I'm considering - or maybe a slightly different one - would be cool. Of course, you may well not have that kind of information for a long time.
That way up and coming stuff doesn't have to dethrone github/slack/etc in order to get seen.