Even though RPGs can vary wildly, there are still plenty of common themes they frequently share—features that a player expects out of your game.
This one goes without saying. Every RPG—and I do mean every RPG—has these basic concepts.
Statistics, or stats, are the numbers that govern all combat in the game. While the actual stats can vary, it's common to have stats such as max health, max MP, strength, defense, and more.
As players progress through the game, these stats also improve. Their character becomes better in a variety of ways, reaching maximum potential at (or near) the end of the game. The exact way in which this is handled can vary, but most games implement experience points or XP that are earned in combat; when enough XP has been gained, a character's level increases, and with it, their stats increase as well.
It's common to have classes in an RPG. A class can mean a lot, but generally it governs what a character's capabilities are and how that character will progress.
For instance, a Soldier class might define that, as an example, a character is able to wield swords, and mainly focuses on increased attack power and defense power as they level up.
Very few role-playing games can get away with not having magic spells or special abilities of some sort.
Generally, characters will have some kind of magic meter that is consumed every time they use one of their special abilities. Additionally, these abilities cannot be cast if the character does not have enough magic (the term for this varies—it might also be called mana, stamina, or power—really, anything to fit the game scenario).