If your game is designed around a grind and around incremental progression with no real variation, then the gap between grinders and casual players will constantly increase.
But if the game is designed so 'real world skill' carries more weight than equipment or stats, then a different kind of gap occurs, but one that is usually less wide, and where newer players can still have a good time and contribute, if proper-match-making occurs. Also, it allows newcomers to get 'up to speed' quicker.
Examples of 'real world skill' includes tactical thinking and decision-making carrying more weight in battle then merely a more powerful sword. Chess entirely depends on real-world skill (several different branches of real-world skill), since opponents are always evenly matched in terms of in-game skill.
Another example of real-world skill is most first-person shooters. I'm not a very good FPS player, but occasionally I'll jump on Modern Warfare or another FPS, and usually while not being the best player (or even in the upper 50% of the team), I can still enjoy myself enough to keep playing, because I can still kill the enemy some of the time. It's not a guaranteed defeat merely because the enemy has epic gear spike armor and is riding a fire-breathing dragon.
Modern Warfare did a really good job of adding advancement and customization, but also keeping things balanced.
Though RPGs should definitely have more customization and more in-game advancement than Modern Warfare, they could stand to learn a few lessons from it as well. Infact, the developers of World of Warcraft, after their Cataclysm expansion, admitted they botched part of WoW's design where it comes to player choice and customization, and then praised Modern Warfare 2's design as better. Ofcourse, it helps that both WoW (Blizzard) and Modern Warfare (Activision) are owned by the same parent company (Activision-Blizzard).
(Despite what it sounds like, I'm not a huge FPS fan, nor am I a huge Modern Warfare fan. Even among FPS's it's not my fav. I just admire the balanced craftwork that went into the three 'Modern Warfare' games)