This question is updated to request additional answers and details in light of information now available in the Dungeon Master’s Guide.
In the released DM basic rules, Lost Mine of Phandelver, Hoard of the Dragon Queen, and the DMG teaser there are wands, staves, and rings that you can use to cast spells.
Do abilities that trigger from casting a spell activate when casting a spell from an item?
Can the character modify the spell using class abilities when casting a spell from an item?
An abjurer Wizard’s arcane ward recovering hit points from a Dispel Magic cast from a Ring of Spell Storing
A sorcerer using Twinned Spell on a ray of enfeeblement from a Staff of Power
An evocation wizard using Sculpt Spell on a fireball from a Wand of Fireballs.
Now that the DMG is out we have some clearer rules on casting spells from items (DMG p. 141):
Some magic items allow you to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn’t expend any spell slots, and requires no components, unless the item’s description says otherwise The spell uses its normal casting time, range and duration, and the user of the item must concentrate if the spell requires concentration.
A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item.
Does whether or not you can apply class abilities to the casting of the spell depend whether the item uses the user’s own spellcasting ability (such as the Staff of Fire) versus an item that comes with a fixed spellcasting ability built-in (such as the Wand of Fireballs)?
Does it matter if it’s something that triggers more or less automatically like Arcane Ward, versus something you have to control, like Careful Spell?