Remind me sometime to tell you about the time my players ran across a Pseudonatural Giant Squid.
Description can go a LONG way to startling players. It's a trick I often use... Keep the mechanics for a spell or monster exactly the same, but describe it in a completely different manner. For example, here's a verbatim account of something that happened at my gaming table a few months back:
My players once ran across an orc shaman. While the characters tangled with his bodyguards, the shaman pulls out his ceremonial dagger, looks at the party fighter and swipes the knife through the air in front of him. On the other side of the battlefield, more than eighty feet away, the fighter can feel a long knife cut open up on his chest, beneath his armor... With no visible marks on his armor at all! A roll of the dice says the fighter just took 15 hit points of damage.
"Don't I get a saving throw?" the fighter's player asks.
"Nope."
"But you didn't roll a ranged touch attack," another player puzzles.
"That's right."
"From eighty feet away."
"Yep."
"And it went right through his armor."
"You got it."
Everyone looks a little worried.
"Wow," the wizard's player decides, "I've got to find out what that spell is and get a copy of it to scribe into my spellbook."
Little did they know that it was just a 7th level Sorcerer casting a Magic Missile spell. At the time, though, they were truly and thoroughly amazed.