Ryan has a good post here about a player in his game that truly roleplays a cleric.
In GURPS or Fantasy Hero the issues he raise are a no brainer to deal with. They are among the many disadvantages one can take to gain more points for advantages, characteristics, and skills. In GURPS terms the character would probably have a Sense of Duty, plus a unique Discipline of Faith.
But he is D&D which doesn't have any game mechanics that benefit characters that start out with self imposed limitations. One way to handle this is how 3.X dealt with issues. Unique classes or prestige classes that give interesting powers in exchange for the limitations imposed by the class
While one can come with new classes for older editions there is an alternative.
Advancement other than levels.
Treasure other than golds or items.
Both boil down to embedding the character within the setting. Allowing the PCs to gain contacts, allies, status and join organizations. When done right the player eat this stuff up. Using this Roghan is not just a 5th level Fighter with a +2 sword, but Rogahn, a Captain of the Mercernary Guild. Or we can have Lukas a senior priest of the Church of Delaquain.
In exchange for the hassles and limitations of dealing with this the players gain lucratives leads as well as access to resources they otherwise may not have.
The free and footloose character still works. They have the advantage of just being able to pick and go whenever they feel like it. But I find that once a player finds something that they are deeply interested in they go with it until the endgame.
For Ryan's Cleric, I recommend that contacts and a path up his temple's heirarchy would be good rewards for the gold he is passing up. Done right the player may wind up being the major driver of the campaign's plot. I find that campaigns revolve around characters with strong convictions or personalities. Simply because that once you start doing A, B naturally follows then C all the way to the endgame.
Derailing Castle Amber
1 hour ago