For those wondering, Ubi didn't have a front-facing name for the engine powering Watch_dogs at E3, but they did talk to me about it. It's using bits and pieces of their various internal engines Dunia, Anvil 2, and Splinter Cell: Conviction's implementation of LEAD (which is based on UE2, which the series started off with).
I got demos of most of these at E3. I saw Watch_Dogs in person three times and the demo went differently each time, by the way, and they covered different ground for the same objective at the end, so it wasn't especially scripted and it's much more impressive in person than any video of it online. Video compression kicks the shit out of the IQ.
Meanwhile, UE4 is doing stuff that UE3 just can't do, particularly with lighting, and I imagine from an efficiency standpoint, it's running global illumination and deferred rendering tech better than the UE3.5 stuff we're seeing in 1313. Again, the videos you can see online of the full demo aren't doing it justice, which is going to be a trend for a lot of games this coming generation. Shit, even Wonderful 101 looks much better in person than it does in any of the videos they've released.
Other stuff: Cryengine 3 and Luminous are very philosophically similar in major ways, particularly in how they mix deferred and forward renderers to focus on each technique's strengths. Weird thing about Agni's Philosophy: first they did a full CG movie of the demo without worrying about the engine, then they rebuilt it in Luminous and did all they could to make them like for like. It seemed considerably more solid than you'd expect from most tech demos, and I don't think it's misrepresentative of what we'll see early next gen. It was pretty impressive in person, and Square worked with programmers and engineers from all over the world, including the west, to put it together.
So yeah. Get excite.