What features does Adobe Illustrator lack compared to Microsoft PowerPoint for making posters?
I believe that Adobe Illustrator has many more features that MS PowerPoint for graphic design and image processing.

The key question is: what will you use those features to accomplish when creating your poster?

I created sever posters in MS PowerPoint and found that it had sufficient features and was easy to use.  The only issues came when trying to print it out, because the output of Mac PowerPoint wasn't compatible with the Windows PowerPoint that the printer used.

In my view, extra-fancy graphic design will not result in better posters.  Instead, what most posters need is bigger fonts, less text, bigger/simpler graphics and images, and headlines + call-outs that guide the viewer through the story.  You can accomplish all these goals using PowerPoint.