I know a lot of people have said similar things, but I'd like to emphasize the difference in experience at the resort... Disneyland has a lot more charm than Disney World does, and there are a lot more details in every aspect of the park, from the lighting and signage to walkways and architecture. My biggest piece of advice to fans of both parks is to take advantage of that atmosphere and spend some time just wandering the park and taking in all of the details outside of the rides, rather than focusing all of your time and energy towards getting to the rides. Disneyland doesn't have the wide walkways that Disney World does, and so it can get more crowded, but it also feels less like a theme park because of this design element. Another note; the Fantasyland dark rides that are in both parks are much more impressive at Disneyland. I think they are slightly darker in Disneyland, and the rooms themselves seem smaller (less floor space showing between your ride vehicle and the animatronics), which for some reason really helps to sell the illusion.
Plus, as I'm sure you know, Disneyland has an entire park that Disney World does not have, California Adventure. There is some ride overlap between the Disney World resort, but the feel of the park is very unique, and with its recent expansion, there are some really impressive moments to be had. I highly recommend a trip to the Carthay Circle Restaurant, and leave yourself some extra time to relax in the lounge downstairs. It does not feel like a theme park, and it's absolutely gorgeous; a truly magical Disneyland-exclusive experience.
I realize I probably sound biased, and I'm not; I love both resorts and would never have to choose between them. When friends who have only been to one or the other ask about the difference, I find the easiest way to explain it is to tell them that Disneyland has all of the detail and charm that make Disney parks feel really special, but Disney World's scale is impressive, and it has the virtue of always feeling like there is something new to see right around the corner; it's easy to walk through all of the two Disneyland parks in a few hours (if you're not stopping to do anything!) and feel like you've seen the resort.