Reaper of Souls : D3 expansion?

[Apoc.ps]

So how does this work in terms of co-op with people who don't have the expansion?
Is it just the 5th act we can't play? I assume we also don't get the new skills.

But apart from that, are all the other important mechanics patched into the base game? (like all the loot system updates, etc.)

[Hawk.ps]

I don't think you can play Adventure Mode either, nor play with the new class.

And adventure mode really is worth the expansion, adds a lot of replay value to the game beyond farming.

All the other mechanics were mostly in since patch 2.0.