Join an Alliance to get access to more rewards via playing the Alliance Wars.
Animations
If you want to check out the best animations, check out Spider-Man, Elektra, Jessica Jones and Gamora. Thor's hammer attack is pretty cool too.
Daily Objectives
Completing the Daily Objectives every day is the fastest way to ensure you ar making the most of game's generosity, unless you are spending money of course.
Healer
Especially in the early stages of the game, always ensure you have a healer in your squad and that they are levelled up as much as possible.
Resources
Always focus your resources in levelling up a small number of characters as much as you can rather than spreading materials across a lot of medium ranked heroes.