First of all, it's not an attack. It's the bright side of life! Not being able to block it could make it stronger.
You have an interesting criticism. Maybe I should consider ultimately adding a little something extra and possibly making it cost 5. But I also have some counterpoints. Mostly, I suppose I should play test this sometime if I really want to see if it works.
In a lot of curse games, it can be really hard to get some golds in your deck. After the curses run out, playing this card gets you more Silvers and Golds while anyone who doesn't buy it is still probably choking on curses and coppers to some extent. If the other players aren't playing Bright Side of Life, then they will have less curses than you, and they might gain a Curse or a Copper on the same turn that you gain a Gold.
It would probably be stronger on boards with no trashing, overall.
Also, you don't have to play the card if your deck already has several curses but your hand has none. The "everyone else gets a gold while you gain a curse and a copper" scenario probably won't ever happen if you play the card judiciously.
In Dominion terms, it's pretty much an attack. It hands out Curses and Coppers most of the time. Yes it does it to you as well, but it's still attacking others. If you want to make it a non-attack for thematic reasons, OK. Then it's still an attack that just doesn't have the Attack type. And of course being unblockable makes it stronger.
I'd already addressed your first counterpoint in looking for a use case for this card -- games that have other cursing, after Curses have run out. In this case, you can make the most of your junky deck and use BSoL as a cheap Explorer, which is fairly decent. But as I said, this defeats the purpose of BSoL being a self-curser. It's actually just a way to deal with Curses, and the self-cursing is irrelevant because you wouldn't want to use it until Curses have run out. That initial penalty on play actually narrows the use case of BSoL -- you don't want to help your opponent win the Curse split even more, so you don't play it until the Curses are gone. You're not going to play it until the Curses are gone, so you won't buy it until the Curses are almost gone. You're buying it so late that you're not going to get much use out of it at all. Maybe if the initial Curse gain was optional, the card would be a little useful. Still niche, which is fine. But again, this defeats the purpose of being a "self-curser" (which I am focused on because of the thread you posted this in).
I note that you say "they will have less curses than you" as if it were good that you have more curses. OK, you can gain slightly better treasure than them... but you have more curses. That is explicitly
not a good thing.
So when does the self-cursing matter? On boards with no other cursing. This is the most important use case for a self-cursing card. And in this context, BSoL is terrible. Again, you
always give yourself a Curse, and you only
sometimes give others a Curse. That's because your opponents can block it with Curses. Even in the very best case, where you gain Gold and your opponents all gain Curses... well, you still gained a Gold and a Curse, and that Curse probably isn't worth it. And again, it's questionable that this is a good case at all because it means you had at least 2 Curses in hand to start, which is pretty bad.
As for your last counterpoint that you don't have to play BSoL if you have no Curses in hand... true. But then BSoL was itself almost as bad as a Curse, since it's a dead card in your hand.
tl;dr:
BSoL almost always hurts you a lot more than it hurts other players. It's too weak at pretty much any price. Its best use case is as a way to deal with already having a deck full of Curses, and for this purpose it is somewhat interesting. But this defeats the purpose of making it a self-curser.
I know it started off as a Monty Python parody, but if you want to make it a playable card you should ask yourself, "what is the purpose of this card?"
If your goal is to make it a viable strategy to give yourself Curses, this card does not succeed. If your goal is to provide a way to mitigate the power of Cursing attacks, then you may be onto something. For the latter, making the self-cursing optional opens up the use cases a bit while still keeping it niche. I'm personally not a fan of Curses-for-benefit concepts, but some people like it.
