Oh, err, guuuys, I think I got what's special about this manga. Took me long enough, but in retrospective, it's understandable that it's hard to reach an answer that is so simple it sits right under your nose all the time. And while looking for some specific or complicated reason, you often fail to notice the basic things.
Please take a moment and try to remember whether there's ever been an actual "Our relationship is %status%" or "From no on we're %status%" moment between these two.
Right, there wasn't. Their relationship is not so much "slow-paced," it's, well...
Realistic. In life (and, by the by, in Japan too, no matter what manga might imply) people rarely choose their relationship's statuses. In real life, you meet someone, be it by a common acquaintance, place where you spend time, or a mere chance, and if you click chances are you naturally start to spend time together.
I think many of you had someone whose company you preferred in school, university, club, or work. I sure had.
Now, maybe less of us have experienced this, but one of the most common foreign misconceptions about Japanese society is that people start dating by confessing their feelings, which doesn't really compare to the western world, where people rarely confess. Thing is, it really is, mostly, a manga trope. People, like in the rest of the world, kinda start hanging out, and before you know it, you start touching and kissing and spending time just with you two.
Now, what I'm talking about, is that BokuYaba nails exactly this. The real world development of relationships. When you just spend time with the people you like and the relationship develops naturally. That's why is seems so easy to relate to. Because it's realistic, truly a rare romance type for manga, since in most hands it would turn out plainly boring.
P.S. Though, I have to say that the story takes some really unrealistic take for the it's sake. 'Slowly developing romantic relationships,' that is.
You know the 3 days rule? That states that you have basically just a few days to start dating with a new acquaintance, and if you don't your chances for this lower quite fast? It has some real actual basis. Ichikawa and Yamada exist in a vacuum bubble where there are no new people in their life. While in reality time is your enemy, and every moment you're not in a relationship is the moment someone else might be trying to take this place. Now, this is something I'm sure lots of you have encountered. You have someone you're kinda interested in in your life, and then suddenly they date some unknown person. That's how it usually happens. When you meet, you have no relationship, so you can create any type. Once you're friends, becoming lovers is harder than from zero.