In the past this only worked in the Finder but with Lion Apple extended Quick Look to work with basically any file reference, from transmitted files in ichat to file open dialogs, Quick Look works (nearly) everywhere.
They also changed the style of the preview window a little bit and added a option to open the file in a matching application.
Its also very useful to get fast information about folders and harddisks.
You can also extend the abilities of Quick Look with plugins, like showing the contents of compressed files or adding support for OpenOffice documents.
Another new Quick Look feature brings us directly to Spotlight.
Already a great feature in Snowleopard, in Lion they extended Spotlight with Quick Look features so you can check the content of files while you search. Just move you'r mouse-pointer over the results and you see a preview immediately.
When you search with Spotlight, you’re actually accessing a comprehensive, constantly updated index that sees all the metadata inside supported files — the “what, when, and who” of every piece of information saved on your Mac — including the kind of content, the author, edit history, format, size, and many more details. Most document types, including Microsoft Word documents, Adobe Photoshop images, and email, already contain rich metadata. And because Spotlight indexes content as well, your search results include what appears inside a file or document, not just its title.
In addition to the Finder, Spotlight search technology is built into other applications in Mac OS X: Mail, Address Book, iCal, System Preferences, Preview, even the Help menu so it shows you where you find the menu items you are looking for.
You even can use Spotlight as a basic calculator.
Quick Look and Spotlight are awesome time savers, instead of opening Photoshop or Office to quick check contents of a file you can get a "quick look" with just pressing the space bar.
Combined with Spotlight you have all the information on your mac available in milliseconds.