1) No. All content (except for Ruby Sanctum) is available.
2) Boost for gear? There are the catch-up mechanics. Trial of the Champion, Forge of Souls, Pit of Saron and Halls of Reflection are all 5-man dungeons that can drop gear that is equivalent (and greater than, in the case of the last 3 on heroic mode) to Naxxramas and Ulduar gear.
There are crafted items from Trial of the Crusader and Icecrown Citadel that are respectively 245 and 264 item level.
Emblems of Triumph drop from all 5-man dungeons and raids before Icecrown Citadel, which allow you to buy 232 and 245 item level gear.
You can get as high as even "5k gear score" before even touching a raid.
3) Rates? 7x EXP, 5x gold, reputation and professions skills.
4) Less played are mostly Rogues & Warlocks, while the most played are probably Warriors and Paladins.