Sense of Sight
John Watson was interesting to watch.
He looked very unassuming. Sherlock figured that this was a conscious decision on John's part. But Sherlock knew the truth. John had deft hands, whether he was fixing tea or examining a body. They were a doctor's hands, quick and precise in their movements.
Sherlock loved to watch John pull on the invisible armour of the soldier persona that he could never quite leave behind. It wasn't that John wasn't a confident man in his everyday life, far from that, but when he drew himself up, back straight, shoulders square, head high, he became someone more. Give him a gun and he became mesmerizing, hands steady, feet planted, something fatal and beautiful all at once.
Sherlock wasn't normally given to such sentimental hyperbole, but he knows artistry when he sees it.
Sense of Hearing
John Watson's voice was uniquely expressive.
Sherlock and John had infrequent but quite spectacular shouting matches. Sherlock had seen John angry at him, and was quite familiar with that tone of voice, the one that meant that Sherlock had done something "wrong" and didn't care and John didn't approve.
John's angry voice and John's command voice were very similar, but the steel behind the commanding tone brooked no argument from anyone. John had even managed to use the voice effectively on Irene Adler. Unbeknownst to them at the time, Sherlock had overheard every word.
But Sherlock was also familiar with John's joking, happy voice as they bantered about cases or about crap telly shows. It was warm, if a voice could be categorized as such, and John's laugh was infectious, making even Sherlock smile.
Sense of Smell
John Watson smells of tea and wool and London and something uniquely his own.
Sometimes he smelled like cologne or toast or the chip shop or, once after a particularly difficult case that almost ended quite badly, river water and the coppery tang of blood. Not his own blood, fortunately, but the blood of the man he'd disarmed and then subdued so the police could take the man away.
Sherlock had wrinkled his nose at the smell of river that clung to John, but John was too high on adrenaline to notice, although John had taken a shower as soon as he'd gotten home. Then he'd smelled like soap and shampoo and clean.
Sense of Touch
John Watson was surprisingly muscular.
Under the coat and the jumper and the shirt, John was still in good shape from the army. The few times Sherlock had touched John, he had felt that through the layers of cloth and wool.
Sherlock, though, could still remember feeling the trembling in John's body as he'd stripped off the Semtex vest that Moriarty had strapped on him. Sherlock almost recoiled at touching the vest itself, trying to keep the coat between his hands and it, and then slung them both as far away as he could at the first opportunity.
Sense of Taste
Sherlock honestly had no idea what John Watson tasted like.
Tea, probably, if Sherlock had to make a deduction. Wool, cotton. Sweat, if he'd been running. Soap, maybe, after a shower. Toast. Chinese food.
They both came in laughing and exhilarated after catching a particularly stupid thief. John was gasping out something that the thief had said to which Sherlock had made a retort that had left the thief gaping at him in open mouthed confusion.
"I swear, I could kiss you sometimes," John gasped out, still laughing, and then froze. Sherlock froze as well, and then hung up his coat and turned to John. "Sherlock, I'm so—"
Sherlock cut him off mid-word by pressing his mouth to John's. John froze again for a moment.
And then John was kissing him back.
A distant part of Sherlock's brain registered that John did indeed taste faintly of tea, but John mostly tasted like John. Another distant, smaller part, was registering how interesting tasting another person was, how it was both touching and tasting at the same time, engaging two senses at once.
Sherlock could smell John's scent that was woolcottonLondonJohn, see that John had his eyes closed, hear John make a small pleased noise in his throat, feel the softness of John's hair under Sherlock's left hand and the weave of his jumper under his right.
But mostly Sherlock was kissing John, because that was something new and different and unique and, somehow, impossibly perfect.