I set the scope of races / classes / levels / etc when I list the game. From then on, it's all about the players.
If they all want to play the Super-Razor-Warriors (as Rouges are Over-Powdered), then that is what they play. If they have a TPK in the first combat as they had no crowd control / healers / buffers / etc, then they will need to rethink their party composition.
Some of the games I'm running have the following makeups:
Level 12 Against the Giants 4e
Halfling Monk Windrider, Half-elf Bard / Euphonic Bow, Eladrin Warlord / Warlock, Genasi Watersoul Swordmage / Swordmage Aegis, Changeling Chaos sorcerer / Primordial Adept and a Human Hexblade / Evermeet Warlock
Level 6 Pathfinder Adventure Path
Human Rogue, Human Paladin, Human Ranger, Human Summoner
Neither of these groups have much healing (the PF group's Paladin is the Tank and the Healer), the 4e group has a couple of heals by the Bard and 2 heals from the Warlord.
I asked the players to make up their characters however they would like them -- I personally enjoy playing Clerics, but not many others do, so I say let the players play whatever they like even if it is Over-Powdered Rouges
If everyone is suddenly a Dragonborn Super-Razor-Warrior, then as it turns out, that must be one of the most common, rather then least common race / class combos on "this" world.
No Dibs -- just what they want