Well, for starters, he often brought up vacationing in Tahiti and it was always followed up with "It's a magical place".
The project involved harvesting tissues from an alien corpse that SHIELD deemed to be worthy for an Avenger, since it had regenerative properties. The side effects were, well. Catatonia, aphagia, psychosis... the test subjects were people handpicked by SHIELD who had terminal illnesses. The regenerative tissue worked very well for them, until they each started to undergo these mental breakdowns. They later found out that each serum of the alien corpse also contained fragments of the alien's memory, and so, they thought the best course of action would be to keep going but to scrub their memories clean, and have new ones put in. The test subjects were realized all well and healthy under new aliases.
Coulson thought the project was just too much and went ahead with shutting it down. Then, he died and Fury decided to resurrect him through the same process.