The problem with Jira is not that it's slow or crusty. It's not slow anymore, and any crustiness is hidden under the hood. Its real problems are a polarizing UI (I like it, others hate it) and flexibility that leans toward bloatware-level complexity. But you can configure it to look/feel as simple as Trello, of course. To beat Jira for me, you'd have to do something revolutionary, like having the entire issue tracker…
I've been mulling over something like that. I don't see why issue tracker, documentation store (wiki, etc), and source code repo need to be separate. Seems more like path dependency more than reasoned choice.