The Big Ops model is that the convention committee plans the convention and then an Ops department runs everything at-con. This is not necessarily a bad way to do things, but it frequently tends to fail badly. It has three main flaws:
First, like any system which moves decision-making up a hierarchy, it is going to be less efficient than one where decision-making is moved as far as possible to the periphery where committee members actually interact with members. And lessons learned at-con don't necessarily make their way back to the planners.
Secondly, by making Ops the center of the con, the other areas are diminished, and talented people gravitate to the power center. We want things like program and the art show to shine, but by empowering Ops we have just moved our best people from the most important parts of the con to administrative roles.
Thirdly, Ops has historically been apt to attract people who want power and prestige for themselves more than they want a great con. (Like security "you don't want people working Ops who want to work Ops.")
The services model has replaced Ops at many conventions.