Save versus Attack Roll

One very interesting thing they did in 4th edition was to remove 'saving throws' and replace them with stat based attack rolls. So in 3rd you are comparing 1d20 + Base Save (as determined by class levels) + Stat Bonus + Random Feat and Magical Bonuses to 10 + Level Based Difficulty (spell level or half HD, actual very similar) + Stat Bonus + Feats and Magical Bonuses. So each factor has something on the other side that corresponds so you have the typical 1d20 > 10 roll. So instead of having the defender roll the 50% to be affected (with all other things being equal) you have the attacked do it. So to have the 4th edition attack roll behavior you add 10 to each saving throw, call it a defense and subtract 10 from every DC and have the attacker roll d20 and add in the DC.

One reason people might not like this is because it takes away the rolling of the dice. Players tend to like to roll dice. It gives them a feeling of involvement. When I GMed Champions we had many attacks that did around 10d6 damage. It became time-consuming counting up those dice and that was one major factor slowing down combat. So I made a handy little dice rolling application to speed the game, but my players weren't into it. They wanted to roll dice because it made them feel like they were in control of their fate. Most players attribute their rolls to either themselves or their dice and rarely to random probability. "I can't hit this guy!" "This die fails every time I need to make a save!" It sounds silly, but imagine playing the games craps where instead of rolling the dice you just read a display. It would be a very different experience and it would take away the illusion of control.

So if players like rolling dice and converting between +10 and +1d20 is easy you could easily switch to a system where players rolled all the dice. When they attacked they would roll an attack roll. When they were attacked by a monster they would roll a defense roll. When casting a spell they would roll a spell attack roll when being hit with one they would roll a save. The problem, you have to track player numbers and monster numbers in a slightly different way. But these numbers don't actually mix too much. A player tracks the numbers on his character sheet and a DM tracks numbers on monster statistics. Conversion is ridiculously simple since it is just subtract ten and add a d20 or vice versa.

One additional advantage I see to this is action points and die manipulation abilities. If you have action points that add a d6 to a roll or some luck ability that allows you to reroll then this allows those powers to be much more flexible.

PROS:
-Greater feeling of control
-Greater power and flexibility for 'die manipulation' type abilities

CONS:
-Two different ways to handle players versus monsters

So I guess this comes down to how hard it is to convert between adding 10 and adding d20. I don't think it would be much of an issue, because it happens when you are figuring out what to write down on your character sheet. If you got to rewrite the rules you could phrase things in terms of everything being an opposed roll where you add a bonus. Then, when you make the roll players add a d20 and monsters add 10.