I'm running the original Temple of Elemental Evil, and here are a few things I've learned:
1. In AD&D, three characters were supposed to be able to fight abreast in a 10-foot wide square, so I tend to convert each 10 foot square into a 3 by 3 square in 4e terms.
2. You can get around the issue of fights taking place in doorways by combining multiple encounter areas into one, especially narrow, twisty areas. For instance, the PCs kick down a door, and the orcs in the chamber call for help from their allies down the hall. That gives the encounter a couple of different "fronts" and makes things more interesting.
3. Don't shy away from fights that are way below the PCs level if something more interesting than a fight is going on. Example: The PCs bump into a squad of 5 orcs patrolling the dungeon. The orcs are all minions, but the PCs need to cut them down before they sound an alarm or whatever. I've found this is a useful approach for wandering monsters.
4. Look at the monsters in the original as a suggestion, not a canonical proclamation. In ToEE, I replaced some of the ghouls with wights, turned a few of the various NPC clerics into gnolls, and added genasi villains/cannon fodder because they fit the theme. It gave me more flexibility in creating monster combinations that were more mechanically interesting.
5. Finally, one of the tricks I learned with classic modules is that they are a lot more fun if the PCs take a strategic approach to them. Give the players a chance to come up with plans, interact with NPCs, and so on. A big dungeon is a lot more fun if the players can interact with it on a level other than clear room A, clear room B, and so on.
In my ToEE campaign, I gave the characters an obvious way to sneak in. They met a recruiter looking for mercenaries to aid the temple of earth. I made it obvious the guy was so desperate that any reasonable ploy would trick him and get the PCs into the temple.
That helped make things interesting, because the characters have swapped between clearing out rooms, allying with the different factions, and learning about the dynamic within the temple. They had more decisions than heading east or west, like debating which two temples to pit against each other, or the best way to trick a high priest into allying with them.