The first thing Luffy noticed about Zoro was his voice. It was deep and rich, even when it was cracked with thirst and hunger and long days spent outside in the heat. Luffy had liked it immediately.
The second thing Luffy had noticed about Zoro was the honesty in his voice. He was blunt and didn't try and cut corners. He had the tone of someone who would follow his own path no matter what. He knew that if he could win the pirate hunter over, the man would be loyal to the end, whenever it might come.
The third thing Luffy had noticed about Zoro was his hair. It was unusual. He decided that he liked Zoro better without his bandanna on. It belonged on his upper arm, not his head.
The fourth thing Luffy noticed about Zoro was his laugh. It was rich and hearty, and always genuine. Luffy didn't care if it was at his own expense, he just liked to hear the older man laugh. Laughter was a way to forage friendship. Luffy wanted Zoro to laugh as much as he could.
The fifth thing Luffy noticed about Zoro was his eyes. They were sharp and alert, always following him. Luffy knew that if something happened to him, Zoro would spot it and stop it before it happened. Zoro could also see when Luffy was fine on his own. That way they were never stepping on each other's toes.
The sixth thing Luffy noticed about Zoro was his muscles. He was slightly surprised that he hadn't noticed before, though in retrospect he supposed he had probably just accepted it automatically. After all, you can't swing around three heavy katanas around, move with that much grace, and not have a lot of muscles. He had just never taken the time to really look before.
The seventh thing Luffy noticed about Zoro was his hands. They were warm and comforting, weather Zoro was catching him, pulling him from the water, getting his attention, or waking him from one of his rare nightmares, Zoro's hands were large and warm, the skin callused and slightly chapped from waving his weights around and practicing to become the best swordsman in the world.
The eighth thing Luffy noticed about Zoro was his lips. Luffy would catch himself paying more attention to his first mate's lips whenever he was talking as opposed to what the man was actually saying. The man would always sigh, or whack him rather gently on his head when he asked him to repeat himself, but he would always do so anyway. That led to the ninth thing Luffy noticed about Zoro.
The ninth thing Luffy noticed about Zoro was his patience. The man seemed to have an endless supply of it, (except for when it came Sanji). Luffy knew he could be a handful, the rest of the crew always told him so. But Zoro always seemed to have time to explain things or repeat himself when Luffy needed him to. And he would always help Luffy if he needed it.
The tenth thing Luffy noticed about Zoro was that he never seemed to look at girls. Even Luffy, who usually didn't notice these things, had to agree with the cook when he said that Nami and Vivi were beautiful, But Zoro never looked twice at them, no matter how they dressed or acted. Luffy had to admit, if only to himself, he was glad, even though he wasn't quite sure why yet.
The eleventh thing Luffy noticed about Zoro was the different smiles he had for everyone. A wary smile for Robin-chan, a sneer for Sanji-kun, a patronizing smile for Ussap-kun, an amused one for Chopper-chan, and a….did he ever smile at Nami-chan? Normally she was demanding money, so he scowled a lot at her. And for Luffy, a special smile filled with fondness and extended patience, one he never gave anyone else. It was a smile just for Luffy. It made Luffy's insides feel warm.
The twelfth thing Luffy noticed about Zoro was his skin. It was a hard tan color. He caught himself comparing their skin colors in his head more than once. He liked the way their skin looked next to each other; The contrast between his soft peach colored flesh and Zoro's skin color which he had decided was the color of fresh baked bread crust. He liked the way Zoro reminded him of food.
The thirteenth thing Luffy noticed about Zoro was the way he stood just a little closer to Luffy, even when he didn't have to, or how he often brushed against him when they walked side by side, or just stood next to one another. His skin was as warm as it looked like it ought to have been. It felt good against his own skin.
The fourteenth thing Luffy noticed about Zoro was how much taller the older man was. He figured that he should have noticed this sooner as well, but Zoro was older than he, so it was to be expected. He wasn't too much taller, though. If Luffy leaned up a bit and Zoro leaned down a little, their noses could touch. He knew because Zoro kept leaning down.
The fifteenth thing Luffy noticed about Zoro was the way his own stomach seemed to do flip-flops when he was around, when he spoke to him, when he told him a secret, when he played idely with his hair when he was trying to wake him up, when he heard him laughing either at or with him, when he caught him staring at him for no reason, when he wrapped his arms securely around his waist, when he pressed his lips to his own, when he waited for Luffy's answer to his question, when he said he would take Luffy over all the beautiful girls in the world, when he smiled at him, especialy when Luffy gave him his answer, how their skin looked meshed as firmly as possible against one another after a day of secret touches and stolen kisses.
Luffy might be slow on the uptake sometimes, and he might not notice things right away, but when he did, he knew he could always count on Zoro to stand by him.