Being a DL guy all my life and a WDW guy during the latter half of my life - I have seen and experienced a lot of the differences and similarities between the two. WDW is a wonderful experience - but it should not be forgotten that without DL there would not BE a WDW. First and foremost, DL was Walt's park. While he certainly was the major force for an eastern expansion / sister park - he never set foot inside the MK at WDW - only walked the ground where things would someday be built. He slept, ate in and very much enjoyed DL.
One of the major comparisons between the two resorts, of course, comes in the overall size. DL itself is just a couple hundred acres, while the entire resort today is about 500 +/-. WDW is over 27,000 - or equivalent to the entire city of San Francisco, or twice the size of Manhattan. Everything is quite spread out, and getitng around takes quite a while on buses, monorails or boats. Not so at DL - everything is within walking distance.
DL set the precedent for how all of the eventual Disney parks would be. DL has a much higher ride/attraction count in that single park than any of the WDW parks - and it's a much smaller park, too. The walkways are smaller and more crowded - but the intimacy can be a great thing - except for the most crowded days/times. Yes the castle is smaller - but the smaller castle also feels less imposing. It's been said that the ancient European castles were built large and imposing as a means of intimidating the peasants - that's kind of what WDW's feels like...Sleeping Beauty Castle at DL does not feel that way all - it feels much more friendly and enjoyable.
At DL POTC is about twice as long as at WDW (and actually much better), and the HM is different too - the stretching gallery is an actual elevator taking guests down to a basement level in order to move them underneath the train berm and out to the show building. Indiana Jones, POTC, HM and Mickey's Toontown are all located outside or beyond the berm - even though their entrances are inside the berm.
The DL hotel is a classic experience - but nothing like the hotels at WDW. It was originally built in the late '50's - and became a Disney property in the '80's by acquisition, it was originally built and operated by the Wrather Corporation - and unfortnately, despite multiple renovations, shows it's age constantly. The Paradise Pier hotel is a Disney hotel by acquisition - it was originally built by others as well. Only the Grand Californian is a Disney built hotel...its' lobby rivals that of the Wilderness Lodge - it's gorgeous.
There are numerous rides and experiences that remain unique to DL that you should check out - Alice In Wonderland, Indiana Jones, the Matterhorn, Pinocchio's Daring Journey, New Orleans Square, Finding Nemo Subs, The First 50 Years film in the Opera House on MS, Storybook Land boats, Roger Rabbit's CarToon Spin, Big Thunder Ranch, Primeval World diorama on the railroad, Mr. Toad, Tarzan's Treehouse (similar to the original Swiss Family but somewhat different), the Parade of Dreams, and of course those awesome Remember fireworks - which far out distance WDW's Wishes.
It's an awesome experience! I hope you enjoy it.