Nothing, absolutely nothing. This really was a fantastic location.
Castro Hotel is excellent, really it is. I've been home 1 day and have already made bookings for July 2015 and September 2015.
Some reviews mention the remoteness of the place - absolute rubbish. It's a ten minute walk to the beach and main seafront area, less if you just want to go into the town/village. It's actually that little walk that makes Castro perfect - close enough to be convenient, but far away enough to avoid the hussle and bussle of a busy holiday town.
The hotel itself is great - typically Greek so simple in its layout and features, but spotlessly clean and very well maintained. Of course if you have luxury expectations, then you've booked the wrong place, but if you want comfort, cleanliness, convenience, and value, then this is for you. And don't believe what some reviews say about breakfast ! It's typically Greek/Continental and is a wonderful way to start the day - if you want a fry up, go to any of the local restaurants who will oblige for a reasonable price. We were perfectly happy with our breakfast !
The staff are charming - they are all family, and you quickly get included in their daily lives if you want to be - if not, they'll leave you alone. Very helpful indeed and always completely courteous, they are one of the main reasons we're looking to go back again, and again. And the owner's home made wine and fig jam are just unbelievable.
Santorini itself has rapidly become my favourite Greek island, and I have to say that Kamari was definitely the nicest place to be based. And Castro was just perfect for us - great pool, didn't see more than 10 people use it at once while we were there, sunbeds and the comfiest deck chairs I've ever used.
In short, fantastic place, and I can't wait to go back - July 2015 can't come quick enough.