node and iojs are merging right now, likely the next, “4.0” release, will be the merged one.
In general iojs/node strives to be very lean, basically a thin wrapper around chrome’s v8 engine. As of iojs and the merged node codebase the main goal is to stay up to date with the latest v8 releases (track what’s stable and released in chrome, e.g. v8 4.4 right now from Chrome 44 will be in iojs 3.0).
Most of the developer-relevant development really happens in terms of EcmaScript language features that then trickle down through the v8 implementation into node, and beyond that it’s really the whole npm packages ecosystem that brings new features and APIs and ways to do things.
The node/iojs guys do focus on optimizing the whole (underlying) thing, but the core project is really not what you should be looking at if you want to understand where the journey is going. The core team makes an up to date v8 engine available to us on the server, and gives a small set of core APIs around that.
Beyond that everything happens in npm modules that are either plain JS or native/C++ and bring expanded capabilities. That’s where all the fun then happens.
Hope this helps with a bit of a clearer overview of node (and iojs).