HoYay! is an abbreviation for 'homoeroticism yay!' - which is why it often retains an exclamation mark. Refers to subtext between two (usually heterosexually identified) characters. Its first major use was in the Smallville fandom, referring particularly to the Clark/Lex sexual subtext.
"From last nights episode I think the directors are seeing the HoYay! goodness in the show too!"