The most noticeable monotony comes in the form of the dungeon. If the players only see endless corridors and rooms, of course they will eventually get bored. A simple solution to this is to introduce "demi-planes." Demi-planes (like Ravenloft) are pocket dimensions that exist outside of other places. BRW Games has a couple of these linked to Castle of the Mad Archmage. DP1 + 2 are great examples of changing the scenery. Before this, even Gary Gygax used demi-planes to break monotony. Isle of the Ape, The Land Beyond the Magic Mirror and others were published from the original Castle Greyhawk megadungeon. Any number of modules could be used in this way, however. If you're playing in Forgotten Realms, any Greyhawk adventure could be a demi-plane in your game.
Next, megadungeons need to be more than meat grinders. Not all (or even most?) encounters should be combats. The DM needs to be familiar with the factions within. Factions should have motives, friends, enemies and wants/needs. Some should be alien and extreme, and others should be recognizable as people eking out an existence in a harsh land.
Looking outside the dungeon, it is important to have locales to be travelled to. A safe haven should be nearby, but not have everything available that a party may need. A resurrection spell should require travel to a major city to find a cleric of sufficient level. Finding a sage to research that demon on level 6? She isn't hanging out in an inn on the side of the road somewhere. She is in an ivory tower in a major metropolis, and it is nowhere near the dungeon itself. The adventurers need to journey to get these major components.
Similarly, there should be quests outside of the dungeon. The sage may give information about the demon that leads to Stonemire, where a sword is stuck in an anvil. This sword must be used to destroy the demon, otherwise it just returns in 3-5 months.
These quests can begin within the dungeon as well. Ancient libraries within the dark corridors hold information on relics sprinkled throughout the world. Sprinkle in rumors and information about things the group has asked for (like a Staff of the Magi or Rod of Lordly Might). This creates the variety that your players may crave.
In Mord Mar, I have several outdoor levels within the megadungeon. As it is a dungeon that encompasses a mountain, I have built both up and down. There are hidden grottos of unicorns, griffins nests on cliffsides, mountain villages cut off from civilization and other non-dungeon locales "within" the dungeon. This also breaks up the monotony without leaving the geographical area.
Why else do players balk at the megadungeon? LMK and I will write a blog about it.