Most video game RPGs have essentially given up on what pencil-and-paper RPG players would consider the roleplaying aspect of the genre and have instead decided to focus solely on being combat simulators. With the majority of gameplay focused on the combat system, the combat system becomes one of the central points of the game. The story, if there is one, serves only as an explanation for why the baddies keep on getting stronger.
Usually. Some stories don't even explain that. Why are the guards in the Great City far weaker than the lowliest grunt stationed at Some Guy's Farm? Who knows.
Here's how to create an RPG combat system that truly sucks.
Base It On D&D/D20
Pencil and paper roleplaying games were designed around the limitations of, well, pencil and paper. Overly complicated math would be too difficult to do by hand, so simple math and simple dice rolls are used instead. This is balanced by the fact that (at least in a good RPG session), there is a human GM to override bad rolls. A good GM knows when to ignore the dice and just let the game progress in a fun direction.
A computer, on the other hand, can only decide based on the rules. The D&D rules are meant as a guideline to creating a fun story-telling game. Combat isn't the ultimate goal of D&D - in fact, quite a few rules involve resolving non-combat situations.
The D&D rules aren't meant to be strict rules that must be followed to the letter, they're merely guidelines. Computers, on the other hand, always follow the rules they're given to the letter. That's what they're supposed to do.
Simply copying the D&D rules into a computer game with no thought to the difference between pencil-and-paper and a computer is a sure-fire way to create a combat system that sucks.
Use A "Full Team" Turn Based System
Another sure-fire way to create a combat system that prevents actual tactics and strategy is to create a turned based system where the player inputs all commands, and then all actions are resolved.
This ensures that players have no way of making appropriate actions to respond to enemy actions that occur during that round - only the round prior. It also allows, in extremely unlucky circumstances, for a monster to attack twice in a row with no chance for the player to take an action in between. Combine this with sufficiently devastating "attack all" abilities, and you can easily create a monster that can wipe out an entire party based solely on an unlucky roll.
To add further insult to injury, don't limit the monsters to deciding on actions ahead of time. Allow them to decide on their actions during the turn. Nothing like allowing a monster to resurrect a fallen foe the same round it died, without allowing the players the same courtesy.
Make Large Jumps In Enemy Strength
Have enemy strength jump several levels, with no indication it's going to. This allows players to suddenly discover that they've moved from a level 10 zone to a level 15 zone when they notice that their attacks suddenly all miss.
For added fun, don't include any monsters between level 10 and level 15, forcing the player to either hope they're lucky on level 15 encounters or grind away at level 10 encounters until they can take Level 15 monsters.
Take An Excessively Long Time To Enter/Exit Combat
Since combat is the primary focus of most RPGs, most play time should be taken up by either watching a battle start or viewing the victory screen.