The one problem I have with alts is not so much learning their different abilities, but making sure that I feel as comfortable playing the game with one as I do with my main. You can never have the same abilities with different classes, but some similarities may help you feel better with an alt.
Map keys and mouse buttons to similar abilities – I mainly play with a Logitech MX810 mouse and a Logitech G13 keypad. Their buttons and keys are all set to specific keys in the game, so that when I want to map them for alts, I just have to put similar abilities in the specific spot on the hot bar. For example, on my mage, my Mouse 4 and 5 buttons are mapped to bar buttons 2 and 8. For my mage, bar button 2 is Arcane Blast and bar button 8 is Arcane Barrage. On my druid, those buttons are Wrath and Moonfire respectively. However, on my rogue, they’re set to Sinister Strike and Eviscerate. On the G13, key G3 is Counterspell for my mage, and Kick for my rogue. They both interrupt the target, so it’s easier to know that the same key enacts a similar talent across different accounts. Similarly, I have Dalaran Brilliance on my mage in the same spot I have Mark of the Wild on my druid. Professions are all in the same places as well.
Load profiles from different addons – One of my favorite addons in Dominos. It’s an addon that allows you to arrange your bars in the game so that you’re comfortable with their position and size. What it allows you to do is save a profile for one character such as your main, and use that same profile for your other alts. That way, all your bars are in the same place across all your characters. Many other addons have global settings such as Titan Panel, but others may need coaxing to get them just right.
Set up the same window tabs – This is something I haven’t been able to figure out how to do automatically, but I have an extra tab in the game for whispers, and a whole window just for guild chat which doubles for World and Local Defense. This way, I won’t miss whispers and world event messages on any of my characters.
There are some settings you can’t copy over such as pets or mounts that one character has that the other doesn’t. You can fill them in with what suits one character better than the other (eg: Nightsaber mounts for Night Elves vs. a horse for a Human).
You may find that keeping your character’s UIs and keyboard and mouse settings more similar than unique may help with how comfortable you are playing an alt.