Your base Walt Disney World ticket will only include one park a day. While there's much to do, you may find yourself wanting more flexibility.
Something I always budget into my trips is the upgrade from a base ticket to a Park Hopper. I haven't been on a single trip without getting it. But for many others it can be a burden on their budget or simply just not worthwhile.
Great For: Longer Stays
During longer stays you start to learn what you like and don't like. Towards the end of your trip, having a hopper will let you close out your trip with a "best hits" day. Check out your favorite rides, parks, food items, etc. before returning to the cruel real world. It's also more cost efficient to add the hopper onto tickets with more days.
Bad For: Short Stays
Some might argue that hoppers are great for short stays, but if you're a new Disney traveler I highly doubt you'll end up using the hopper if you have less than 4 days. Magic Kingdom alone can take 2 days and you really don't want to be running in circles for your short trip.
Great For: Flexible Touring
The biggest thing for me with the hopper is that it removes the ball and chain of only visiting one park per day. There are many days where I show up to one park and the crowds are worse than predictions. Instead of having to bite the bullet and have to deal with them, I can just hop over to another park (and hope it's not that crowded either). I highly recommend the hopper during the Epcot festivals. There are so many great food items and it's worth skipping a meal and hopping over to Epcot each day to try something new.
Bad For: Budget Travelers
A park hopper is going to balloon your budget big time. Even if you buy your tickets from a discount seller like Undercover Tourist, you're talking a significant price jump of over $50 for each ticket. For one or two people that's not a huge hit, but for families it definitely is. The hopper is a nice feature, but it isn't worth blowing your budget.
Great For: Returning Trips
Headed back to Disney? Chances are you've done most attractions already. Depending on when you visited last, much may have not changed. A hopper is a necessity if you've "been there, done that."
Bad For: Slow Movers
Park hopping is incredibly time consuming and requires lots of movement. You'll have to walk to a transportation option (bus, monorail, boat, or your car), head over to the next park, and go through security and the entrance gate all over again.