I think the main focus for Valve will be updating the toolchain. From what I understand it's pretty bad and still has some legacy stuff from the Quake days. TheHollowNight can talk more about this. So I think this might be the thing that can be a real generational leap.
I'm not sure they're going to go crazy with physics. People are speculating destruction based on the pics, but I don't know.
Streaming is probably also guaranteed. No more load screens. I mean, come on. Would be embarrassing if otherwise, me thinks.
What I'm really looking forward to is seeing the animations. How much will they go toward motion capture, but still maintain their stylized, expressive nature. The ragdolls definitely need an update though.
Most definitely will be fixing the Art pipeline workflow. Compared to UDK/CryEngine or any other modern engine, it's embarrassing. Writing manual qc files you have to manually drag onto a batch file to compile a model is not something you should be doing in 2014.
It will likely use a similar Content browser like Unreal judging by the slides, where you can properly organize and filter assets & reimport them with a right click if the FBX has been modified. I'd be surprised if they still keep the vmt/vtf texture file system. it's all archaic to be honest.
Valve have progressively been moving into more heavily modeled environments with each new game. I'm sure they will keep the legacy brush editing tools they have (it really is the easiest to pick up). But they are bound to be mostly a model based engine like Unreal. This would require new streaming tech, since BSP visibility is mostly controlled by the designer in Hammer with areaportals and vis brushes. Streaming requires more precise and dynamic PVS system to cull meshes out of the players viewpoint whilst streaming in new stuff. So yeah, hopefully we'll get a load-free game!
A feature I'd personally love would be simultaneous real-time map editing. Currently you can break down maps into sub-maps called instances, meaning multiple people could work on portions of a map and update to a server without overwriting progress (pretty standard). Having asynchronous editing would be a dream, seeing people in the same 3D space building the world with you.
Also model instancing. Where you take one unique instance of a mesh & duplicate it as many times as you like without affecting the data or memory footprint, since it uses the original's. Pretty sure that's a DX10(?)/11 feature.
Model scaling. How in the hell has Source never supported this? bahhhhh.
Deferred Rendering. Probably a given since DOTA 2 has moved onto that. They will probably have some Global Illumination solution implemented to make up for the lack of baked lightmaps & radiosity/light-bouncing. Would be amazing if it was realtime GI, but no idea, to be honest.
Every feature DX11 allows. Volumetric lighting/shadowing, particle lighting/shadowing, real-time reflections, maybe Physically Based material system? Deferred and PBL go hand in hand. Along with DX11, it is highly likely to be supporting OpenGL 100% since that's been Valve's push with the Steam Machines/OS.
Editor UI to be customizable, window tabs & docking elements how you please. The 3D viewport will probably display lighting/effects in realtime, or at least, an approximation of what it will look like. (Curse you lighting preview!). Please be Ctrl+G to play.
Mostly likely has upgraded physics/destruction? Maybe on the scale that cinematic physics achieved, but dynamic (since those are actually animated simulations exported from a 3D program).
God I dunno haha, so many things in the engine I'd love and Valve have ignored in the past. I just hope they don't break the toolset as frequently as they did with Source & screwed over lots of mod teams.
All I'm sure of is some sort of backwards compatibility with Source to allow content migration to Source 2, much like they did with GoldSrc to Source.