A reminder that the Pack Mules were an "E" at Disneyland at one point.
Determining "ticket level" is like General Relativity: You have to define from whose point of view you're using.
Historical Paper Tickets
During the time of Paper Tickets, the ticket level was originally determined by the cost of the attraction. You paid more for attractions that were expensive to build. And so, the E-Ticket represented the highest price at the time for a ride on the most expensive rides.
However, over time, the paper ticket system started to be used as crowd control. The various versions of Astro Orbiter are just spinners, and thus, started out as a B-Ticket. But because they had such a low capacity, they were bumped from B to C then even a D-Ticket. Alice in Wonderland went in reverse... as a popular new ride, it started as a D-Ticket, then went to C and finally, a B-Ticket.
Post-Paper Tickets: Guests
Guests learned that the E-Ticket rides were the ones with the longest lines. Partly because they were very popular since Disney put a lot of money into it. But also the rides with low capacity and long lines were bumped up in 'grade' for crowd control. So, for the average guest, an "E-Ticket" is any ride that they really like and has long lines.
Post-Paper Tickets: WDI
For Imagineering, they use the graded ticket level internally, but, AFAIK, there is no written guidelines of what qualifies a ride to be an "E-Ticket." It seems that for Imagineering, the level of immersion has to be top notch from outside, through the queue, and with with a ride system which is cutting edge with cutting edge SFX. Consequently, a ride that may have been an "E" decades ago, would not be considered as such if built today. So, sometimes the 'grade' of a ride has to be graded on a chronological curve.