Dwarves have been mentioned in a few posts.
When I started my 4e game, I told the players that I wanted to play as per the core rules defaults, and that within that constraint anything goes. I also said that each PC had to have (at least) one loyalty, and also a reason to be ready to fight goblins. So one player's PC was a dwarf fighter. He explained that, among the dwarves, one didn't come of age until killing one's first goblin in battle. But his PC, despite serving in the militia for many years, had never achieved this feat and hence never come of age - whenever the goblins attacked he was on kitchen duties, or carrying a message to the back lines, or whatever. The result was that he had become something of a laughing stock, as many younger dwarves had already come of age while he was still a "child". So one day, with his mother's blessing, he set out into the world to find a goblin to kill and thereby prove himself . . .
That PC backstory satisfied all my stipulations - a loyalty (indeed, a whole cultural grounding) and a reason to be ready to fight goblins and it didn't contradict anything in the default setting of the core books.
For much of the campaign the main significance of the details of this backstory (as opposed to the general dwarfiness) was that one of the other warrior PCs (voiced by a teasing player) would refer to the dwarf's lack of genuine fighting skill, penchant for being on latrine duty, etc. When the PC's reached paragon tier, however, and were travelling in the foothills of the mountains where the dwarves live, I decided to bring it into play in a different way. By that time the PC had developed a cleric multi-class, and the player chose a cleric paragon path (Warpriest) for the PC. At some relevant time - probably after an extended rest - I described the appearance of an angel, from Moradin, with a message that a group of dwarves needed aid. The angel led them (or gave directions - memory fails) and the PCs, led by the dwarf, arrived on a scene of carnage - a group of dwarves scouting far to the west of the Dwarfholme had successfully repelled a band of hobgoblin attackers, but many had been killed and many of the survivors badly injured.
Of course (as I narrated) the dwarf PC recognised many of these dwarves from his time in the militia. And they recognised him, and mocked him - What are you doing here? An angel told us that aid would come, so why are we getting the latrine cleaner? There were obviously a variety of possible responses here, but the player chose to demonstrate his prowess and assert his status as no longer a "child" dwarf but the most powerful champion of Moradin in the land (mechanically, in the context of the skill challenge, a skill check - Intimidate? Diplomacy? - modified by the expenditure of an appropriate encounter power exemplary of martial prowess). The check succeeded, and so the PC's standing among his fellow dwarves changed suddenly and dramatically - the rescued dwarves acknowledged him as their leader, and willingly followed him. Sadly many of them died not too much later when the hobgoblins attacked again, with reinforcements, while the party was staying in a hill village, and the hobgoblins' dinosaur knocked over the house where the injured dwarves were sheltering. (But this encounter did result in the only dino-riding ranger I've personally seen in play - the PC elven ranger killed the hobgoblin beast tamer and took control of the behemoth for himself - it was rather Legolas-like!)
One of the surviving dwarves - Gutboy Barrelhouse - who had initially been the most derisive of the PC, appointed himself the PC's herald, and has held that role since.
It is very hard for me to envisage a better series of story developments that would have happened by "policing" or overriding the player's initial presentation of his PC.
