What is the point of increasing/decreasing attributes in increments of 5 instead of 1, and then have all the formulas divide by 5, 10, or 15? Just divide everything upstream and downstream by 5 and get it over with. Yes, saying you have 25 Intelligence sounds nice, but sucks and looks worthless when it only affects things by 5, 2.5, or 1.6 points (25 / 5, 25 / 10, or 25 / 15 respectively).
Games should never use powers or other exponential formulas. Yes, it looks good on paper and it keeps people sucked into the game longer, but it is also equally frustrating to be stuck at a certain point for too long. I don't like games that take days and weeks on end to level up once or twice once you get above level 30, 40, or 50. Formulas should always use a constant modifier when it comes to costs, whether experience to the next level or gold to buy an item or upgrade.
When will games learn that 10% is a worthless bonus to a new player? If you're doing 2-8 damage and you have a 10% bonus, that means you're only doing 2.2 to 8.8 damage instead. A more note-worthy bonus for a new player would be a static bonus such as +2 to damage given or -3 to damage taken, but will prove just as useless to a higher level player. Another idea is to combine the two types of bonuses for later levels of equipment.
Monsters should be plentiful in abundance, variety, and rewards. They should also be well-balanced so that fighting one monster doesn't reward too much more than another of equal or greater difficulty. It is always sad to see level 50 players "farming" a level 10 monster for gold, items, or especially experience.
Getting the next best weapon, armor, or spell shouldn't cost an arm, a leg, and a kidney, much less be impossible. Having to fight a level 25 monster at level 20 to get a level 15 item is just wrong. Nor is it right to cost half of a player's savings just for one of the four items a player needs. Seeing a level 30 player with level 15 or 20 equipment because everything else costs too much or is too hard to get is pathetic.
Fights that last three minutes for one battle had better give me a bucket load of rewards or be pivotal to the story. I don't like sitting there clicking "Attack" every 15 seconds. It's repetitive, wastes my time, and isn't fun, It's a boring monotony and the only reason it's put up with is the prize just out of reach beyond the fight.
Experience should be given for every attack in battle, not just winning the fight. Experience should also be based on the similarity of level between the fighters so that fighting someone too low or too high results in less experience than fighting things around your level. Getting beat up by something 20 levels your superior should not result in massive experience bonuses (What have you learned, other than that getting hit hurts), and you shouldn't be able to kill something that far above you in the first place (Abuse of the bonus system, both by the players and the creator. The bonus system should give an edge, not a monopoly).
Luck as an attribute has got to go. A spell or an item may grant more or less favorable luck, but it should not be quantified or controlled by character development. Luck is essentially a random factor that gets used everywhere; when characters are allowed to add to their luck it becomes something that everyone has to have and thus becomes redundant.
Plentiful potions are a staple of any adventure game. If they can't be bought at the local 7-11 then they shouldn't be too hard to find or make. Also, potions don't necessarily have to be drank to be effective; splashing them on the wounded part is a variable alternative.
== And now we get more toward theory ==
Health points are very unrealistic. Instead use a system where stamina and skill is used to determine attack and defense. As the fight progresses stamina goes down and allows a chance for attacks to hit based on skill vs skill. Health itself should be minimal and it should be taken away based on how critical the hit is. Minor scratches would be 1% to 10%, moderate hits would be 10% to 25%, major hits would be 25% to 50%, and critical hits would be 50% to 75%. Anything more than that would be considered an instant death blow.
Another idea would be for each body region to have its own health status and anything that uses that region will have to check on its health. Using a left handed weapon while the left hand was at 50% health would reduce the effectiveness of an attack. This would mean that all items would have to be self-aware of the user but that wouldn't be that hard. A self-aware game environment would go a long way to making things more interesting.