Real-time editing of content and assets without the need to rebuild the whole level again just for 1 asset change.
The luminous engine is the closest I've seen.
There's two features demonstrated in that video, both are a variation on what Phantom calls "live link" in this thread:
The first feature is you see them tweaking variables in-game. Every game engine should have this to some degree.
Some are better than others -- e.g. one engine I used recently allowed you to tweak values like this, but not save/update them, so you'd have to simultaneously make the same changes in the editor in order for them to be permanent.
There's also different interfaces for doing this tweaking, the main options I've seen are:
- An on-screen display, like in the video.
- A special mode that launches the game as a window inside an editor tool, with regular windows forms / toolbars / etc for making changes.
- An external application connected via a network, which can display the game's internal data and allow you to make changes.
The second feature you see in the video is a live-link with Maya. This is a special variation on option #3 above, where you write some special plugin/Mel/Python code, which connects to the game via a network and syncs data between the DCC app (Maya) and the game. This is a great workflow because the artists get to use the tools that they're used to, and any tweaks they make are actually being made to the source art files too, so they're preserved.
I've personally never been lucky enough to work with an engine with this feature, but I know Melbourne House had it in the 90's between XSI and their PS2 engine (before I worked there), and Crytek has it to some degree (animations can be synced between Maya and game for tweaking).
As mentioned in the above-linked thread though, there's other variations on this idea besides "live linking". IMHO, "live link" should be a mandatory feature in any game engine, to allow for fast iteration on tweaking, and in my experience, it has been present in the last 4 engines I've used.
Two out of the last 4 engines I've used have also supported "asset refresh" on top of "live link" -- where the modified asset data can be recompiled and reloaded while the game is running. This is less instant than "live link" -- taking from a frame to a few seconds, instead of instantly -- but is much more general, allowing larger changes to be quickly iterated as well (e.g. a whole texture, a whole mesh). IMHO, this is a required feature for a big engine, as the reductions in iteration time are immensely valuable over the course of a project. Every engine should have this feature going forwards.