This. I think Shunsui's always had that serious side to him. He's not as giddy as someone like Kisuke but he's just really calm and doesn't like fighting. Plus he's always had that thing for women. He was definitely the right decision, Unohana seems quiet and fairly unbalanced (almost schizophrenic if you want to go that far), and Ukitake is too frail when you need someone on the front lines.
The 'serious side' is a recurring thing among the senior captains, they're all relaxed people but they fight dirty and they don't play fair. Shunsui attacks people mid-conversation, Ukitake doesn't even tell Starrk his shikai and lets him figure it out for himself, and Unohana randomly pulls out blades to stab Zaraki during their battle. But looking at the other captains, someone like Ichigo will hold back for a bit and try to find out someone's emotions through striking swords with them, Rose tries to make battles artistic, and the rest of them give a 10 minute speech on how their Bankai works before they attack.
This is probably why Shunsui is one of my favourite characters, along with the other senior captains. Fight, get the job done and return to SS for a nice cup of tea.