Steam Community Beta Access/Steam Trading Card Beta

nothing

nothing unless you're mad

Well, In theory, Trading Card beta should still give old badge for participation in beta.
The experience you received from badge will go toward account experience (community levels).
Once you reach level 5, you will be able to get mystical cards by voting for games on Steam Sale (at least it was like that last time).

Aside from helping in making big "card/idle farms", I'm not sure if these gifts are worthy. Maybe somewhen people will start hunting them "just incase of their rarity" in some small collector communities? If collecting will grow, then they will become expensive, but I don't really believe in that.

I think they can be collected and used to craft badges.
You gain them by playing games, they can be in idle.
If you have some cards you can sell them and get profit.