Sight-reading at the Piano. Some pianists enjoy it. Some pianists hate it, but everyone can do it and everyone can get better at it. Once in a while I hear a student pianist say "I'm not good at sight-reading music" and for some reason they don't expect that to change. With time, patience and deliberate sight-reading practice it will change.
In elementary schools, its not strange to hear five-year-old children say, "I'm not good at reading." Having just learned the letters of the alphabet it's not surprising that the children find it difficult to read. In a year or two they will have no trouble reading. Same with anyone learning music. Music notes are nothing more than simple letters. Letters combine to make words. Words combine to make sentences. Sentences combine to make stories, ideas, books, etc. Music notes do the same. They combine bit by bit until they form musical pieces. Sight-reading at the piano is a matter of learning the "alphabet".
When I was in a community college and my professor asked me to sight-read a choir accompaniment, I told him that I couldn't do it without practicing first. He told me to try anyway. I did, and he was not satisfied with my reading. He told me that I should practice sight-reading everyday for at least half-hour. I took the critique personally and practiced for more than an hour a day, sometimes two hours until my head hurt from figuring out the notes. I realized why sight-reading was so difficult for me. It was because it took a lot of mental energy. I could practice scales and arpeggios for hours, and I wouldn't get tired, but sight-reading for thirty minutes would drain me. After practicing sight-reading for hours, after playing hundreds, thousands of different pieces, sight-reading has become much easier. It actually became a really fun pastime. Sit down at the piano, read a new piece, and enjoy a concert that you never heard before as a performer.
Having said all that, I want to provide a new resource for practicing sight-reading. There are a lot of resources out there, but there can never be too much sight-reading music.