what kind of things i like in an rpg depend on how i feel. sometimes I just want to go all like actiony and play a game like Mass Effect, Secret of Mana, etc... but then others I'll be in the mood for Baldurs Gate or FF. Though, I have to say my favorite battle system of all time is probably the one in Heroes of Might and Magic 2.
a cool "element" I liked is having artifacts, which will boost up a given stat or whatever. though, my favorite part of HOMM2 is the way it handles leveling, you choose between 2 things each time you level up, whether it Necromancy Advanced(Ups the amount of skeletons a Necromancer makes after winning a battle) or estates basic(Ups the amount of gold you get per day.) idk it's just a realllly good game, and it's even more fun playing with friends. battles can be very challenging when you start a map, but it usually gets easier and then the computer wins because it cheats! it also makes you use a lot of strategy, as most battles can't be won just fighting blindly.
another cool thing is if your hero and his troops are really strong, it would say something like "The enemies are awed by the power of your forces, and scatter away. Do you wish to pursue them?(yes/no)". Or, sometimes they would offer to join your group if you diplomacy skills are high enough, but declining would result in them getting mad and attacking you.
it did so much, yet it was simple. plus the graphics are so damn charming and the music is catchy. so yeah basically everyone make games like this.

anyways, sidequests are great if they're actually important and feel like part of the story(Chrono Trigger for example!) and open/linear stories are both good, as long as the story is good.