Examples of E-tickets at WDW today would be:
Expedition Everest
MIssion: Space
Test Track
Soarin'
The triple mtn whammy - Space, Splash & Big Thunder
Rock N Roller Coaster
Tower of Terror
others could qualify - but this is the kind of thing the tickets represented...the best and most thrilling attractions would have the e tickets. Each ticket book came with a set number of A, B, C, D, & E tickets, and when they ran out - the thought was that people would use the remaining tickets that they had already paid for to experience the "lesser" attractions - and consequently spread out the visitors throughout the park. People could always buy more tickets if they wanted to...
An example of an A would be the Horse-Drawn Streetcar and other little things like that.
Back in '82 or '83 (or thereabouts) when Sally Ride, the first female astronaut, came home from outer space, she said that the journey was a "real E-ticket ride." America knew exactly what she meant, and it was the ultimate compliment to Disney!