I've been set into rant mode by reading the following in a Microsoft training manual:
A DTS package can perform a plethora of tasks...
The author appears to adore this abstract collective noun, as it appeared half dozen other times in the same manual. Of course, it was always used incorrectly.
Plethora is, of course, a favorite 10th-grade vocabulary word, which probably explains why it is sophomorically bandied about so much.
When you think about it a little further, there are no legitimate uses for plethora in ordinary conversation. In every place you might use it, too many is a better choice of words. Although the user of plethora hopes to impress people with his or her vocaulary, such a usage almost always muddies the meaning.
You'll always see the construction a plethora of something, since it's virtually impossible to speak of particular plethorae. Although this construction is supposed to mean too many, it's usually used (incorrectly) to mean very many. Unfortunately, this may be another losing word usage battle, similar to the situation with hopefully.
You might use plethora for comedic emphasis. The only true plethora I've encountered in my life was the selection of flavors at the Vancouver gelateria Pseudo_intellectual took me to when I visited him. Which is hilarious if you've seen the gelateria in question.
Of course, if you use plethora in front of your grandmother, she'll be so impressed she'll probably buy you an ice cream cone. But if you're really that manipulative, use surfeit instead. That way, you can have two scoops and chocolate sprinkles. Which should be more than enough.