Roger had a blog post yesterday about how the first level or two of a typical megadungeon is often pretty boring, with scanty treasure guarded by rats and centipedes and the occasional small band of goblins. His point, of course, is that this paradox of a boring introduction to much more interesting levels below is due to people slavishly copying the old megadungeons.
I don't think that's entirely the case, since most of what we know of old megadungeons has been pieced together recently. The old megadungeons weren't published. Also, there's something to be said for easing players into the dungeon; you want the first couple levels to be not too difficult, so that players can prepare for the greater challenge below.
However, I think you can maintain the approach of stocking low-level common monsters in a mostly-empty level if you the level interesting in other ways. One is to include hints of what's below. A single stone head that seems to have been broken off a statue, but it's done in a more realistic style than any of the statues on the level (hint: medusa or something like that down below.) An adventurer corpse with a partial map and strange things scrawled on it, like "AVOID THE EYES!" or "ROD GOES HERE." A pelt of some unknown creature, still pretty fresh.
Another thing is to stock it with resources, a few obvious (wooden items that can be turned into makeshift torches or bars for doors,) but many that are only potentially useful, without a clear and obvious pointer to that use (bag of salt, box of marbles.)
What I'd like to explore sometime is a third way to make it interesting: "semi-tricks". We're all pretty familiar with tricks, but a lot of the trick ideas floating around out there (fountain of random potions, statue that grants wishes or utters curses) are pretty major effects. I'm thinking of a "semi-trick" as being something designed on the same principal, but without as big an impact.
More on this some other time.