The Internet is littered with PowerPoint templates that fall roughly in 2 categories, I would classify them as follows:

1. "Potpourri"-style
2. "Adobe InDesign"-style

Potpourri. These are the slide templates that have been around since the mid 1990s.

• Not very pretty: a very "PowerPoint" design with gradients, bevels, shadows that is often trying too hard
• Hard, if not impossible, to integrate with prescribed corporate template files
• Filled with cliche icons and stock photos 
• Sites offer "tens of thousands" of slides, but they are highly inconsistent across designers

Adobe Indesign. Recently, print/web designers have been branching out into the world of PowerPoint: creating very pretty designs that look a lot like the finished product that is created in Adobe InDesign. But there is a reason that InDesign-style presentations do not work very well for everyday business presentations:

• They tend to ignore the way PowerPoint works with template slides: instead "hard coding" text boxes, shapes, and images on a blank page. This is very hard to customise as a non-designer, and it is impossible to fit into a corporate PowerPoint layout
• They mostly are designed around paragraphs of text. Headlines are big and bold, but text is incredibly small. From a distance the grid of images and paragraph text looks pretty, but it is impossible to read.
• The compositions are dependent on exactly the amount of words, pictures, paragraphs that are presented in the template slide. Have more text, one more option, less text, and you have to redesign an entirely new grid layout.
• Custom fonts make porting the source files between devices hard (most people don't even bother and in no time the presentation will end up in Arial).
• The layouts are all about presenting lists, or blocks, there is no visual movement that is important in business presentations: cause-effect, pros and cons, trends, sequences. 

I am not linking to examples of the above categories since I do not want to single out individual template providers.

The SlideMagic app and template store try to create a corporate visual language that will not look as pretty as the "InDesign" slides, but has a good enough design. What I give away in terms of design, pays back big time in terms of practicality.

Cover image via WikiPedia