Game developers, from indie to the gigantic, can now access Unreal Engine 4 with a $19/month subscription (plus 5% of revenue from resulting sales). This is a much different model from UDK, which was free to develop games with their precompiled builds until commercial release, where an upfront fee and 25% royalty is then applied. For Unreal Engine 4, however, this $19 monthly fee also gives you full C++ source code access (which I have wondered about since the announcement that Unrealscript no longer exists).
Of course, the Unreal Engine 3-based UDK is still available (and just recently updated).
This is definitely interesting and, I believe, a response to publishers doubling-down on developing their own engines. EA has basically sworn off engines outside of their own Frostbite and Ingite technologies. Ubisoft has only announced or released three games based on Unreal Engine since 2011; Activision has announced or released seven in that time, three of which were in that first year. Epic Games has always been very friendly to smaller developers and, with the rise of the internet, it is becoming much easier for indie developers to release content through Steam or even their own website. These developers now have a "AAA" engine, which I think almost anyone would agree that Unreal Engine 4 is, with an affordable license (and full source access).
Speaking of full source access, licensees can access the engine at Epic's GitHub. While a top-five publisher might hesitate to share fixes and patches, the army of smaller developers might share and share-alike. This could lead to Unreal Engine 4 acquiring its own features rapidly. Epic highlights their Oculus VR, Linux and Steam OS, and native HTML5 initiatives but, given community support, there could be pushes into unofficial support for Mantle, TrueAudio, or other technologies. Who knows?
A sister announcement, albeit a much smaller one, is that Unreal Engine 4 is now part of NVIDIA's GameWorks initiative. This integrates various NVIDIA SDKs, such as PhysX, into the engine. The press release quote from Tim Sweeney is as follows:
Epic developed Unreal Engine 4 on NVIDIA hardware, and it looks and runs best on GeForce.
Another brief mention is that Unreal Engine 4 will have expanded support for Android.