Why is Paypal terrible? (Honest question, I find it to be a great service. Am not US-based, if that matters.)

They are very anti-seller.

They lock accounts on a whim and require extensive, unnecessary documentation to get your own money back. They recently settled a class-action suit because of this: https://www.accountholdsettlement.com/

They nearly always side with purchasers and reverse charges despite ample evidence supporting the seller.

Its fees are just sheer theft. I just used it for collecting money from friends for a small paintball event I want to organize, and it kept 10%! Seriously, people sent 5 euros, and I got 4.48, for no reason I can see.

Not to mention that currency conversion is a complete ripoff. I avoid PayPal like the plague.

Paypal's fees are 2.9% + $0.30 USD, which is pretty standard among commercial-level (and you were using it commercially, otherwise they'd have sent it as a gift) processors that don't require a merchant account. Braintree is 2.9% + $0.30 USD; Stripe is also 2.9% + $0.30 USD if you're under $1m/year.

I don't know anything about commercial stuff. All I know was that I wanted my friends to send me $5 so I could pay the paintball guy, and PayPal kept 10% of that.

Fees, and its not that great for the recipient. Better for those sending. Being a bit hypocritical though, I use it myself.

Because creditcards are not in wide use everywhere? There's more to the world than just the United States.