I'm a bit late to this thread, but wanted to chime in because I too have been sorta tortured by my attempts to imagine what Guilds is all about with nothing to go by but its title.
I imagined at first that there would be guilds based around existing card types. Because Dominion only has a few groups of card types that are definitively labelled on the cards (action, victory, treasure, reaction, duration, looter... I think that's it, but I don't want to look it up) I imagined that the guilds would be based around this, like Labor guild, Banker guild, Prospector's guild, etc., and I know that if this is the case Donald has better names for these guilds than I do. But if that IS the case, then I kind of think that there will be guilds cards of a single copy each (if my list of card types is complete then it only needs one more card type to reach seven, which could theoretically be a new card type introduced in guilds), and either all are out or none. And I kinda think they will have a permanent effect. I know most of this has been postulated in previous posts. I don't want to take credit for it, but to those of you who have made these predictions, I agree with you.
So, this thing comes out in spring, right? That's a long time to speculate. By the time it comes out I imagine this community will have designed an entire fan expansion based on our collective theories about the real one.
P.S. I love the Tulip Farm card idea. I would love to see how this plays in real life. You obviously don't want to buy the last one unless you don't have any or are just trying to destroy the other player's strategy, but you also don't want to let your opponent have only one or two.