Text (WL+errata): Until end of turn, target player can't cast instant or sorcery spells, and that player can't activate abilities that aren't mana abilities. ; Draw a card. [Oracle 2009/07/08]
It only affects player's right to announce things. It does not affect anything which was already announced but has not yet resolved. [D'Angelo 1997/06/30] In other words, Abeyance cannot be used to counter a spell or ability. [Duelist Magazine #19, Page 34]
The player can respond to the casting of this spell by playing instant spells and abilities other than mana abilities. But after this spell resolves, they cannot announce any more this turn. [D'Angelo 1999/07/10]
Never prevents mana abilities from being played. [WotC Rules Team 1997/10/06] This means that you can tap lands for mana and play abilities of cards like Llanowar Elves which are played as mana abilities.
Does not prohibit abilities which are not activated, such as continuous abilities of cards like Crusade, triggered abilities, or declaration of attackers or blockers. [D'Angelo 1999/07/10]
Abeyance affects spells based on the spell's type and not on whether they are played when some other type would be legal. So, it does not prohibit spells that say "is played as an instant" or "can be played whenever you could play an instant" if the spell's type is not otherwise prevented. For example, an enchantment that could be played when you can play an instant is legal under Abeyance. [WotC Rules Team 1997/10/06] Similarly, a Summon spell played via Winding Canyons is legal. [Aahz 1997/09/09]
Does prohibit the abilities of cards which are not in play. For example, the Ashen Ghoul's ability can not be played while it is in your graveyard. [WotC Rules Team 1997/12/18] [Duelist Magazine #22, Page 27]
