If you are only going to Disneyland for one day, try to focus on the rides that don't exist at WDW. In addition to the Disneyland one-of-a-kinds, there's a few rides that are noticeably superior or different than their WDW cousins. I suggest trying to fit in the following Disneyland attractions that will be new or very different to you:
E Tickets-
Finding Nemo Submarine Voyage (Either first thing in the morning, or 10 minutes before the park closes to avoid the long line)
Matterhorn Bobsleds (a Disney classic!)
Indiana Jones Adventure (superior to Dinosaur)
Pirates of the Caribbean (vastly superior to WDW's)
Space Mountain (vastly superior to WDW's)
D Tickets-
Pirate's Lair on Tom Sawyer Island (an impressive extreme makeover for the old island)
Enchanted Tiki Room (original 1963 show that is fabulous)
Sailing Ship Columbia (a Disneyland exclusive)
Davy Crockett Explorer Canoes (weekends only in February)
Roger Rabbit's Car Toon Spin (get a Fastpass!)
StorybookLand Canal Boats (Walt's charm and artistry everywhere)
C Tickets-
Peter Pan's Flight (superior to WDW's, but do it in the morning)
Alice In Wonderland (a Disneyland exclusive)
Casey Jr. Circus Train (a fun and zippy way to experience StorybookLand)
Pinnochio's Daring Journey (a Disneyland exclusive)
Mr. Toad's Wild Ride (a Disneyland exclusive)
B Tickets-
Disneyland Opera House (great model of the park circa 1955)
Tarzan's Treehouse (Tarzan evicted the Robinson family back in '99)
You'll also want to spend some time exploring areas of Disneyland that are much different from WDW's versions at Magic Kingdom. Mickey's Toontown is much better at Disneyland. New Orleans Square is many peoples favorite land at Disneyland and is worth exploring the shops and alleys. Fantasyland is also more detailed at Disneyland, with a half dozen additional rides that simply don't exist anywhere in WDW.
Rides you can skip at Disneyland because they aren't as good as WDW's versions are the following:
Splash Mountain (plotline is frenzied and hard to follow at Disneyland)
Many Adventures of Winnie The Pooh (not as good as WDW's)
The rest of the Disneyland attractions are fairly similar to the WDW versions. Star Tours, Buzz Lightyear, Jungle Cruise, Haunted Mansion, Big Thunder and Autopia have different queue setups or a different overall look and feel at Disneyland, but the general ride experience is fairly similar. Innoventions is fun if you have a techy in your group or it's raining, otherwise don't bother. Small World is very different at Disneyland, but it will be closed for a 10 month rehab in '08.
The following WDW rides don't exist at Disneyland;
Stich's Great Escape
Carousel of Progress
PeopleMover
Monsters Inc. Laugh Factory
Country Bear Jamboree