Without going into the details (about which I know too little in the case of Node.js), the main difference is that Node.js supports only concurrency without parallelism while Akka supports both. Both systems are completely event-driven and can scale to large work-loads, but the lack of parallelism makes it difficult in Node.js (i.e. parallelism is explicitly coded by starting multiple nodes and dispatching requests accordingly; it is therefore inflexible at runtime), while it is quite easy in Akka due to its tunable multi-threaded executors. Given small isolated units of work (actor invocations) Akka will automatically parallelize execution for you.
Another difference of importance is that Akka includes a system for handling failure in a structured way (by having each actor supervised by its parent, which is mandatory) whereas Node.js relies upon conventions for authors to pass error conditions from callback to callback. The underlying problem is that asynchronous systems cannot use the standard approach of exceptions employed by synchronous stack-based systems, because the “calling” code will have moved on to different tasks by the time the callback’s error occurs. Having fault handling built into the system makes it more likely that applications built on that system are robust.
The above is not meant to be exhaustive, I’m sure there are a lot more differences.