It makes a great bread slicer. (Kidding)
If you do any -and I mean any- text editing, do yourself a favor and get TextWrangler.
Apple's Mail client is pretty good, and worth setting up to at least try.
I use Appigo's Todo (which syncs to my iDevice) for task management - it's the only one I've found that I really like.
+1 for Evernote.
Whatever you do, don't use Adobe Reader. It's a steaming pile of **** and security holes. I've yet to come across a PDF that Preview can't handle, and my PDF load isn't light - forms, documents, annotation.
MS Office 2011, IMHO, isn't worth paying for. For a clean $20 per app (sold individually), you can get Apple's iWork (Pages, Numbers, and Keynote), which have handled almost everything I've thrown at them, and have the advantages mentioned earlier. They can open and save MS things fine.
If you're a programmer, learn Xcode.
EDIT: Forgot Alfred. Set up a hotkey, then whenever you need something (an app, simple math, complex math, a definition...) it's only a few keys away.