Scrolls stack with consumables. You can have a consumable buff (flask, battle+guardian elixir), one food buff, and one scroll buff, so long as the scroll does overlap with a buff. You can't have arcane intelligence and use a scroll of intellect at the same time.
Potions are 15-sec consumables that stack with everything because it is a temporary, short-lived buff.
For elixirs, you can have 1 battle elixir and 1 guardian elixir at one time, or just 1 flask (counts as both).
Potions share cooldowns with other potions.
One food buff at a time. Eating a new food overwrites the old food.
Scrolls are overwritten by the same, or higher level scroll of the same type, but you can have multiple scroll types.
Example:
I use Scroll of Agility II, Scroll of strength III, Scroll of Protection IV. All 3 are active at the same time. If I use a Scroll of Agility III, it will overwrite my Agility II, but a Scroll of Agility I will not.
Armor and weapons can have 1 permanent enchant and 1 temporary enchant. Enchants, LW Armor kits, Engineering item buffs (like the belt enchants) count as permanent enchants. Blacksmith socketing their gloves and bracers do not count as an enchant, but you can only have 1 extra socket on your gloves/bracers.
Temporary enchants are Mana Oil, or a shaman's Windfury Weapon, or a rogue's poisons.
The blacksmith example above is, I think, the only Profession that does not affect the chance to get permanent enchants. Blacksmiths can socket their gloves AND get a permanent glove enchant. Scribes cannot put their own shoulder inscriptions on top of vendor-bought shoulder inscriptions.