Source - http://news.softpedia.com/news/Destiny-Weapons-Can-Be-Used-by-All-Classes-Complex-Upgrade-System-Is-Included-436168.shtml
First up, the weapon system in the game will not impose any class-based restrictions and characters can have at least two weapons on them at the same time, no matter the choice.
"There are lots of character elements that are exclusive to player classes. Each breed of Guardian will fight and move and grow in unique ways. The weapon arsenal, however, is a free-for-all of firepower and mayhem," the studio said about the new gun system.
What's more, the game will allow players access to the Tower, a special area where they can upgrade their weapons and gear based on the raw materials they'll get during battle. The drop system will be favored towards what players need, so they won't get useless items they'll never use.
"The Tower will be a place where you can restore and upgrade your weapons and gear. Some of the loot you capture on your missions will be items that you can equip or wield. Other treasures that you reap from the action you sow will be raw materials that you need to upgrade your inventory. The best thing is that your rewards will be pre-destined. The fates know what you need, and your fortunes will favor the brave."
-taken from article