She might see herself as rational and efficient, but to a lot of people she comes across as rude, insecure, and worst of all: horribly inconsistent.
She practices a sort of "off with their heads" style of management and loves the sound of her own voice. She will often interrupt people before they have had the chance to communicate even a fraction of what they have prepared for meetings, often spewing forth cascades of unpleasantness as she argues against what she thinks the messenger intends to say.
Very often the upshot of a meeting is that people are more confused as to what her intentions are than they were before the meeting.
She often sends teams in one direction, and then later, yells at them for going in that direction and sends them in a new direction. She seems to have very weak memory for what decisions she has made in the past. People rarely (if ever?) point this out to Marissa.
She also seems to be quite terrible at managing her staff. She is unable to delegate meaningfully, and if something is OK'ed by one of her reportees you essentially have nothing: there is a complete disconnect so you need to get it from Marissa directly. You also need to verify with her frequently to make sure that you catch it when she suddenly changes her mind.
Due to her foul mood, people rarely, if ever, speak up and tell her when she is being an ass or that she is contradicting herself. Again. The reason is that Marissa will have people people removed if they annoy her. I've seen this happen. I've seen people get thrown off projects just for saying something that annoyed the cupcake princess in a meeting.
That she supposedly schedules 70+ meetings/week is sufficient evidence of this. I suppose this is one definition of a "flat" organization though.