Absolutely nothing.
From the tediously overblown login procedure, to the antiquated and unintuitive layout, to the unhelpful yet lengthy list of message options, to the fact your session can time out without warning as it doesn't recognise you typing a message as "activity", to the fact they alert you to a reply by sending an email with subjects like "17" or "3".
But what I hate MOST OF ALL is their pathological inability to answer a question correctly. For example I asked them "when does the promotional rate on my [account info] end?" It's a pretty straightforward question, they have all my account info and they still managed to get it wrong, with an added bonus of the most flowery, frilly language around it. There's polite and factual (all I ask for) and then there's is gushing and wrong (what I get).
(I loathe loathe loathe Santander, but they keep giving me a huge wedge of cashback because I pay my mortgage from a 123 account I set up for that purpose, so I guess my principles have their price!)