For a starter in leaving bits on here about good fit and bad fit -- the armhole.
There are a lot of preferences when it comes to styling. You must decide the lapel widths and jacket length and pants drape. But when it comes to the armhole and your suit there is a good and a bad.
The standard armhole you see at shops today is created by using a ratio to determine how people's bodies are shaped. Because the armhole is so big, you end up with a batwing effect that makes the suit hard to wear. The fabric pulls around with you as you move.
A fit like this always tends to ruin my day, especially when I decide I want to crawl under a train when escaping from an assassin that has recognized me on a trip to Prague.
With a more fitted armhole, you have more mobility and the fabric doesn't drag around your body, it moves with your body.
Now with this armhole cut, not only could escape with ease, I also have a chance of winning the fight that might ensue on the top o the train car. I wouldn't have to remove my jacket. It would move with me and I'd look less disheveled in the end.