I went shopping for some formal footwear with a friend recently. He didn't know the difference between open-laced and closed-laced shoes. I found this article, which sums the difference up nicely.
Open-laced (Bluchers) oxfords have the sides of the shoe sewn on top of the front part. When laced, the shoe looks like it's built out in segments. Front, top, side, back, etc.
Close-laced (Balmorals) oxfords are drawn together by laces sewn under the front part of the shoe and then closed over its tongue. The result is a shoe that looks as if it's made from one piece of leather.
While open-laced shoes are not informal, they're not formal enough to wear with a suit. This, like most other fashion rules, may seem arbitrary, but that's how things "are."