So what do you do to make these iconic monsters more scary?
I know what you are talking about - I recall in 3rd Edition, there were a lot of melee monsters that even the Fighters were unwilling to get close to, since a full round would drop any character straight to dead! Nowadays, they know nothing has they ability, so they always wade right in.
...but I don't necessarily think that is a bad thing. 4E doesn't want characters to just pop. They don't want monsters who hit so hard that no one can stand next to them. They don't want spells that just wipe out half the party. And I never really enjoyed playing a dude in full plate who got to sit back and throw axes at giants, because he knew if he walked into melee he would die.
This doesn't mean fights can't be hard - it just means the fear tends to kick in after a round or two, as the party watches their hitpoints rapidly vanishing, faster then they can handle.
If your party is shrugging off the attacks of Great Wyrms or Beholders and instantly beating them down, then I suspect you have a very effective party fighting against easy or standard encounters. The Beholder has some brutal eye rays and is firing off half a dozen each round! A Great Wyrm White Dragon
can drop a completely healthy fighter in a single round.
I'm running a game currently at 10th level, and had the party fighting an NPC Elite Troll Warlock. He dropped the party barbarian in the first round of combat (cursing him, spending a Daily, then spending an Action Point for his biggest Encounter Power.) That gave the party serious pause.
But you don't see combats that start with one spell dropping half the party, followed by the surviving half nuking (and killing) the enemy, and the combat being over by the second or third round.
The other thing to keep in mind (which some others in this thread have mention) - scary is now determined by the challenge the party faces, not what that challenge is. A level 10 solo dragon isn't supposed to be more difficult than a group of level ogres. A level 15 lich isn't supposed to be scarier than a level 15 wizard.
If you want a fight to be scary, make it higher level than the party. You don't need to go absurd - just recognize that a standard fight is a standard fight, and a hard fight is a hard fight, and you shouldn't expect some enemies to be
inherently more powerful than other enemies of their level.
Finally - in 3rd Edition, the big bad lich was often a single enemy in a fight (if he was of the party's level.) So compare his 20d6 Fireball to the output of 2.5 Liches in 4E - and more importantly, compare it to their actual area effect spell, which does 6d6+7 damage. Average damage times 2.5 = 70 damage, identical to the average damage from 20d6!
So, let's say we have a 4 person party up against 2 of these guys. Round one, the Liches drop 50+ damage on the party, and Action Point to throw their Necrotic Orbs that do another 3d8+7 to one PC... and stun them.
Sure, they might not hit with all this, but you are looking at taking away half the party's hp and stunning half the PCs. That is pretty brutal, especially if those powers start recharging.
I think the real issue isn't even the dichotomy of 4E vs 3rd Edition, it is a difficulty in actually weighing the numbers. 6d6+7 looks smaller than 20d6... but it still significant, especially since it is coming from only one portion of the encounter. Keeping that in mind can go a long way towards realizing monsters can still be a big threat, without needing to one-shot PCs whenever they win initiative.