i agree with much of what has already been written - i totally agree with Wayne regarding space mountain - i MUCH prefer the WDW version..
regarding DL/DCA vs WDW......there really isn't any comparison......nothing comes close to WDW...
however, every disney fan should try DL/DCA at least once (and also DLP, DLHK, TDL/TDS).....and there are some attractions at DL that are better....
it also has some that WDW doesn't have, like Indiana Jones - which is kind of like Dinosaur in AK, but IJ is far superior.....
california is a place unto itself.....and certainly worth a visit, even without Disney.....but as everyone has described - it's BIG and BUSY.....the roads are crazy.....don't expect the compact simplicity of orlando....orlando is teenie tiny compared to LA......
as for DL itself vs the Magic Kingdom (which is pretty much the direct comparison), there are pros and cons to each park, although overall i've always felt that DL is the prettier park....except for the castle which is about 1/100th the size of the castle in florida...ok...i exagerate....but it is soooooooooooo short you'll be shocked....my kids wouldn't even look at it....every time we walked by they'd say "OH NO, THE CASTLE... DON"T LOOK AT IT".....(and they were about 18 and 16 at the time)..
also, IMO, fantasyland at DL is better than fantasyland at the MK....it's prettier and has several rides that just don't exist in the MK (my favorite is the tiny circus train that goes through the storybook area - i could ride that thing all day - call me crazy)....
the year we did DL, we also did WDW.....we started at WDW for 3 weeks (plus a 4 day wonder cruise)...then headed to DL/DCA for 4 days, which was enough time to completely cover both parks....we stayed at the grand californian, which was nice since you have direct entrance into DCA and DL is very close by....
then after 4 days in anaheim, we flew to vegas for 4 days, but that's another story...
i would definitely go for DL/DCA at least once to experience it...
it also has the original Tiki Birds (if you remember them)...and yes, their pirates is far superior to the one at the MK....(the ride is twice as long)...
and you can eat at the Blue Bayou, right inside pirates...
oh....one thing that we thought was fantastic at DL......the fantasmic desert buffet on the balcony of the gallery....only 20 people sit up there and you have the best seats in the park for fantasmic....and you get all the deserts and drinks you can consume....it was lovely....and we really really enjoyed all the people we met that evening...
enough of a ramble for one day...
i'm coming back to add, that i would also go to all those other things everyone has mentioned...
we didn't, because that trip wasn't geared as a california trip....but if you go just to california i would definitely include all the places that have been mentioned....however i wouldn't use the disney hotels as my base....i would stay at one of the disney properties while visiting disney, and then stay at a hotel or hotels that are more convenient for the other attractions i want to visit (e.g., actually stay in san diego for the sea world, san diego zoo portion of the trip)...being caught in southern california's famous traffic jams is not my idea of a vacation....