I think what made Macross Plus for me at first was the music, which every Macross title is dedicated to at some degree. It was essentially Yoko Kanno's big break for US and I still stand by my analogy that she is much more famous in US than in Japan.
Other than that, it was the simple love triangle of Macross Plus that hooked me and it's characters, as well as my semi-love for flight. Defining moment was Isamu and Myung in the forest after he escapes from the hospital.. or the very start with them Isamu and Guld cycling down the hill for the glider with Myung watching - you knew you were watching something special after that. The OAV just had so many moments where you were like "whoa" throughout it and I think Shoji Kawamori has not done anything as grand since (we'll see how Aquarion turns out though).