@noiob The domain name.
...which wouldn't be a big deal if it weren't that domains (especially TLDs) are increasingly a market, with changing prices and rules. The .cat NIC could, at any time, decide to start enforcing the Catalan language requirement, and there'd be nothing I could do about it.
(Note: the domain name was not my idea; other people set up TootCat, and I took over hosting when their priorities changed.)
This isn't a huge complaint, but it really actually would be nice if federation were based on a self-generated key instead of a domain you have to pay for.