How does the + thing work?
Do I just subscribe somewhere using +hello and it shows me those separately somehow?
Is there a limit on these?
I've quickly tested. You don't need to setup anything, if your email address (free or paid) is, for ex, pladio@proton.me, you can give pladio+watch@proton.me to someone / some site, and you'll receive it in the same inbox, with "to = pladio+watch@proton.me". So there's no limitation, you can use as many of those as you want, and you can use filters to do whatever you wish (you get only 3 labels & folders with the free version).
You can reply and it keeps the same address with the "+".
With the paid versions, you can define several aliases, which can be anything if I understand correctly (for ex. you could define another email "guardian.nonsense@proton.me" or even with your domain name if you have one). I'd be more tempted to use that as an address for someone I don't entirely trust, because the free "+" solution above is known and shows your private email address (I'm not sure I see the point to be honest).
I saw the email tracker detection feature, it's nice. Marketing emails often include a link or even a simple image with a custom name, so when you open your email, the image is fetched and the sender knows you've read the email. Proton detects that.
The VPN is very easy to use. The free version gives access to servers in US/NL/JP (perhaps it's not the same everyday or for everyone, I don't know). The bitrate was high enough.
EDIT: I see Myrthos has answer in the mean time, but it's apparently a little different with Proton.