Personally, I avoid Arial. Since it's the default for PowerPoint, it shows you put no effort into the slides. Also, fonts have personalities. For example, Arial's is “stable, conformist” (credit: Nancy Duarte – http://www.slideology.com). Seth Godin has an interesting take on the issue as well: http://sethgodin.typepad.com/seths_blog/2006/07...

Other things to consider are how you are using the text. Is it a title? Is it for copy? Title text gives you more freedom to use a decorative font while copy requires a focus on readability. Once you determine the personality you want reflected, consider:

- x-height.: Typically, the higher the x-height, the more legible it is. Of course, there is a point where it can be too much.
- form and counterform: Thin forms combined with large counters make for text that's hard to identify. The same could be said for thick forms and small counters. A good balance lends itself to legibility.
- width and height: Text that has balance in its width and height are more legible. Text that is imbalanced in favor of either width or height are less so.
- stroke width and height: Again, balance helps legibility.
- the age of the audience members. http://www.aiga.org/content.cfm/typography-and-... is a great source of information.

I would argue that you'll be hard pressed to attain visual impact with Arial or Helvetica and Arial is, in my opinion, a pretty crude Helvetica impostor. Another thing to consider as well is that the Web rules don't necessarily apply to slides since the audience isn't sitting at the screen reading large amounts of text (if they are, then shame on you).