We are bi-coastal folks, going to both WDW and DL each year and have our own thoughts on the parks. DW and I, along with our eldest, are all CA natives, and I worked at DL in high school and college. I enjoy both Disney resorts. Both have their own charms and their own positives - and of course, both have their own negatives too. DL is the original, and is "Walt's park" - since it's the only one in the Disney pantheon that Walt actually walked in, slept in (his apartment is still above the fire house on MS), and interacted with guests in. Sure he picked the WDW site, and walked the property - but he died years before the MK every opened. Both parks are special - and have incredible stuff to offer.
There are definite differences - of course, with just 2 parks vs. 4, no water parks, and only 3 on-site hotels vs 22... no bus system, etc, etc. The sheer size alone of course is quite notable, but that doesn't mean DL is anything inferior - in fact, the DL park is in many ways superior to MK. They both have great things to offer. Hydroguy has some great tips in a sticky at the top of the board. Be sure to check those out.
As far as park to park goes, DL although smaller in acreage, has a larger daily capacity than MK simply by nature of having more attractions, shows and entertainment on a daily basis. DL pretty much stays open for regular guests MORE hours in any given day than MK does, too. Throughout the peak periods DL might be open til 11p or Midnight - whereas pretty much closes at 7, 8 or 9 pm with the once weekly EMH feature. Too many hard-ticket events at MK compared to DL.
Most WDW vets immediately notice and comment on the size difference in the castles. This is natural - but it doesn't mean that DL's is inferior... in fact, Sleeping Beauty Castle is actually more charming than Cinderella Castle. CC is quite imposing, considering that you can see it quite well more than a mile away at the TTC - but it doesn't mean it's better - it's just bigger. YOu can certainly see it from all over the MK. The highest point in DL is the Matterhorn - the first iron tubular roller coaster (not to be missed!)
Many of the rides and attractions that you are familiar with from WDW originated at DL proper. Soarin in Epcot originated in DCA. You will find a few WDW transplants at DCA, too (ToT, MuppetVision, Turtle Talk with Crush, etc). While Wishes is a nice fireworks show at MK, DL's Remember Dreams Come True show is smack-down incredible. The Parade of Dreams is also an awesome parade, and much better than what's at MK. (Rumor has it now that in '09 DL's parade will be packed up and shipped off to FL to replace the long-tired parade there). Some rides remain completely unique to DL... don't miss Alice in Wonderland, the Matterhorn, the Davy Crockett Explorer Canoes, Indiana Jones and the Temple of the Forbidden Eye, the Finding Nemo Submarines, Pinocchio's Daring Adventures, Roger Rabbit's Car Toon Spin...
DCA is also quite small - but has its' own charms about it. It can be considered bland and boring, sure, but it's not terrible. The Grizzly River Run is a fun rapids ride like Kali River Rapids at AK.
All in all - if you go in with an open mind, and not try to see what is and isn't exactly the same as Florida, you really can have a nice time. The DL resort really isn't a week long experience, but it could easily be 3 or 4 days. If you have a week, then there are lots of other neat things in So. Cal to experience - certainly some of the beaches nearby are some of the best in the state.
The DL hotel dates back to the late '50's and early '60's. It was NOT built by Disney at all...in fact it wasn't even owned by Disney until 1988. It's been updated, but still feels old and tired. The Paradise Pier Hotel also was purchased by Disney in the mid '90's. The only on-site hotel originally built by Disney is the Grand Californian. It feels very much like Wilderness Lodge, AK Lodge and the Grand Floridian in the sheer size and scope of the lobby. It's a very nice hotel, in the Craftsman style of early 20th century California. Naturally it's the most expensive of the 3 DLR hotels.
There are a vast array of hotel offerings near the DLR (many are just across the street) and within walking distance. Since DL was built in the '50's and the surrounding community grew up around it, it is definitely boxed in. There are lots of shops, restaurants, hotels and motels directly surrounding it. One of the primary reasons Walt wanted to build the Florida property - to get away from the clutter that surrounded DL.
Have fun exploring the differences and reading what tips others post...