This is a Technology Preview release -- a code snapshot that was released to show what features are planned for release 3.0, and possibly to get early feedback about them. It has several exciting new features that make it well worth trying. The first thing you'll notice is that there are several new example simulations...
... and the old ones have been updated, too. It has also been made easier to browse them, via the "rundemo" program in the samples directory. One of the simulations, TicToc (based on Ahmet Sekercioglu's TicToc tutorial) has been explicitly written to facilitate learning OMNeT++, as it provides eleven models with gradually increasing complexity and with source code comments explaining every step.
Once you launch the first simulations, you'll notice that models now have several new means to display their states, to make it easier to understand what's going on. These new features range from changing icon colors to displaying status strings and "bubbles". Models of networks are now laid out automatically on the canvas, so you don't need to manually tweak graphics coordinates or use GNED. Some actions that have not been previously animated have become animated now: direct message sending, modules calling each other's C++ methods.
While debugging your simulations, you'll also appreciate the new powerful object search dialog that will let you quickly find (via various filters) the objects you're looking for, be them lost messages, state variables in some modules or statistics, and you'll probably like the sliders too where you can directly change the animation speed.
The above is only the surface however. A very important addition is the model documentation tool opp_neddoc, which can extract HTML documentation from NED files and complete it with diagrams and cross-references. It links to Doxygen-generated C++ source documentation as well, which -- especially after turning on Doxygen's Source Browsing option -- provides a very efficient way of presenting comprehensive model documentation from overview down to the last details.
Another feature that can have a big impact on how people use OMNeT++ is dynamic loading of NED files. It basically means that if a NED file doesn't contain simple modules, you can bypass the NED compilation (nedc) and C++ compilation phase. You can load the NED file directly into the simulation program, and then use the components (networks, compound modules) defined in the NED file as if the NED file was compiled into the executable. This enables new possibilities. For example, if you write all building blocks (simple modules) necessary for queueing simulations, you can create a simulation executable that contains all of them -- and after that you can throw out the C++ compiler and delete nedc altogether! The executable will contain all C++ code you need to simulate any queueing network -- the necessary NED files you can write separately and dynamically feed them into the queueing simulation program...
A lot has changed under the hood too. You (hopefully) won't need to care about this, but the object ownership mechanism in the simulation kernel has been redesigned both to reduce memory footprint and to increase runtime efficiency. "Heapflag" and "stor" variables in cObject have been sent to retirement too (if you know what I'm talking about -- don't worry if you don't). Simulations have become about 3-8% faster (depends on the exact code of course.) Bytes gained per object (cObject) have been used to create a storage area inside the object for short object name strings. Setting short object names (about less than ten characters) will be faster because no heap allocation takes place, no malloc or operator new is involved, and won't consume any extra memory (compared to no object name at all) which can be extremely important with large simulations. (I have heard about large simulations where messages and dynamically created gates had to be created without names to save memory -- from 3.0 up there won't be a reason to do that.)
The new parallel distributed simulation feature is probably less practically important to most users (it still experimental and mostly used for research), but it has important side effects: with its implementation the simulation scheduler has been made pluggable. This means that real-time, hardware-in-the-loop and distributed heterogenous simulations can now be implemented in a much cleaner way, and especially without modifications to the simulation kernel.
All the above is still not a complete list of the news. Where you can find out more: from the WhatsNew notes (attached to the download items as description), and (after downloading) from the ChangeLogs in various source directories (include/, src/sim/, src/tkenv/, src/utils/). To date, The User Manual has only been updated with the description of opp_neddoc.