Despite it's theme, I can't help but think Unicorn when I look at this card. It just suits their tempo game, where there's a race to knock down provinces as fast as possible.

I love it

That will be frustrating to play against!

The added economy of not having to worry about defense for an attack can be great, and guaranteeing your opponent misses one break can be a big tempo swing for a lot of decks.

Yes, I think pretty much every deck will be considering this. Mostly though, I guess you'll be wanting this card if you don't normally put your Earth province under your stronghold. So... Phoenix as well?

I'm using it in phoenix, you know, DISPLAY OF POWER

I've found myself putting in Public Forum in any deck that I run Pilgrimage.  (Or at least considering it in those decks - but since it's a new card, "considering it" pretty much means putting it in until I have a chance to play it a few times at which point it may come back out again.)  Basically, if I have another choice of stronghold province (Ancestral Lands, Entrenched Position, and Pilgrimage are the usual standard choices), Public Forum seems much better than the other Earth provinces.  The biggest problem is that the standard Earth provinces are too easy to farm for ring effects.   With Public Forum you can easily force a break if you think that will be better than allowing ring farming.  And you don't have to use the interrupt if you don't want to.  And against decks that are trying to actually break your provinces rather than farm rings for dishonor (or honor, I suppose?), it's frequently just as good to make them break it twice as it would be to make it really hard to break once.

I even swapped out Shameful Display for Pilgrimage so I could run Public Forum in my Lion deck (the alternative being Fertile Fields under the stronghold, which isn't a horrible choice).  Not sure if it will stay that way or not, but this province just seems really good.

The biggest boon I think Public Forum offers is that it allows you to effectively not defend and still potentially outpace your opponent's breaks.  At 3 strength there are few situations where the opponent won't break in two conflicts, but in an aggressive deck that might be enough to push you into the lead.  I think any agro deck that's trying to rush out a victory in turn 2 or 3 will like this very much. 

Any clan that is trying to defend their provinces to play the long game (Winning by incremental advantage) and desires to get multiple triggers from actions on provinces might not find this province choice optimal.  It is effectively a safe province for your opponent to farm rings.

I agree with Phillos - it depends on deck. In some it's excellent as it allows you to outpace your opponent. In others you really don't want to be playing anything in your central 4 that they can attack without consequences.

Don't forget also, that since the province isn't breaking, cards like For Greater Glory can't be played.