Because of the dining plan, most of the table service places book up in advance and aren't able to accomodate walk-ups. A few places have a blanket no-walk-ups policy, but even those that accept walk-ups will often stop taking them after a certain time of night if they are booked.
'Ohana is a prime example; they will accept walk-ups until they are booked solid - which happens pretty often. But even when they accept a walk-up, the wait is usually 90-180 minutes.
The easiest places to get into without a ressie are the buffets. I typically have no trouble getting into Hollywood and Vine or Liberty Tree Tavern as a walk-up, or if I stop by the restaurant earlier in the day and ask for a dinner ressie. But I'm solo - it's a lot easier for me to get in someplace than a party of 3 or more.
For the most popular places, an advance ressie is pretty much a must these days, especially if your party is more than 2.