I'm okay with this preview. As Mercule said, it's revealing the entire design of the Deck of Many Things. It's a return to the 1e definition, using the 4e card designs. It's a "legendary" level item, and the effects of the three cards shown make it very clear that it's going to be a potentially campaign-breaking item (like 1e) rather than a watered-down "balanced" item (like 4e). Some people will be happy about that, some people will be unhappy about that.
I'm on the fence. The return to 1e power levels means that this is an item that, as the DM, I will have to use very carefully. There's certainly a place for it at my table, but only under certain circumstances.
As for complaining that it's not really a preview unless they reveal all the cards... The definition of a preview is "an opportunity to view something before it is acquired or becomes generally available". But D&D rules aren't entirely a physical product. If they previewed the entire item, then it is already "generally available". An HTML or PDF copy of the entire Deck of Many Things is not a preview; it's a release. This preview tells me everything I need to know about the design of the 5e Deck of Many Things, while still leaving me some surprises for when I buy the DMG. I'm fine with that.