
In the last version, PowerPoint was limited in how big a poster it can make (56 inches, according to here). Some conferences give several feet of space, and PowerPoint couldn't reach the large sizes.
PowerPoint is also wretched at typesetting and handling complex layouts. It’s harder to change even basic paragraph settings like line spacing in PowerPoint than Word. Perhaps the fatal flaw is that the heart of any poster layout should be a consistent grid, and setting up a grid in PowerPoint is very difficult. Consequently, I see many posters where I’m willing to bet that no two items on them are actually aligned. They’ve been roughly kinda sorta eyeballed.
Yet using PowerPoint is so entrenched for making posters that printers provide PowerPoint templates for clients and actually encourage the abominable practice.
Strangely, many who use PowerPoint have an much superior tool at their disposal: Publisher. It’s part of the standard Microsoft Office package, but not many people are aware of it. It uses many of the same commands and logic as the other Office software that people know, so its lack of popularity is all the more surprising. Publisher has its limitations, but the improvement over what one can do in PowerPoint is huge.
(Original version posted at NeuroDojo.)