First something I don't like:
Appropriate Theme. Please do not have the fate of the entire kingdom/continent/world resting upon the shoulders of the 1st level adventurers. That bugs the heck out of me. I cannot tell you how many times I have ran acros this: "The kingdom is in dire peril from [X] and so the king calls for heroes. Your characters are given the task of saving everyone in the kingdom. (Adventure for 1st level characters.)" AAARRGGHH!! Who on earth sends children to deal with the big bad, when you have knight-lords, magister-wizards and high-priests to send?? So please, don't do that. If it is an adventure for low-level characters, make the theme appropriate to them; village/town-based, basically smaller-scaled. A great example of that is the Basic D&D adventure called B11: King's Festival: "Someone has “borrowed” a Cleric, and without him, the fabled King’s Festival cannot go on. Unfortunately, it looks like the orcs have him, and your characters must rescue him. A great learning adventure, King’s Festival provides players and DMs with a valuable introduction to fantasy role-playing in the land of Karameikos." [There's more, but I am sure you get the gist.] It is about a cleric and a festival for the town of Stallanford - not even the capitol of Karameikos - not saving the world from ultimate doom. Because really, where does a group go from there? After saving the world, how exciting does finding a kidnapped princess seem?
Other than that, just have excellent maps, art, and a combination of overland and dungeon* locations, particularly ones that give great detail to the local setting and are appropriate to the local environment. (No desert creatures in the middle of the frozen tundra - unless, of course, there is a reason.) I also really like it when the "random" encounters can be used as clues in the main storyline, or a side-story the PCs can get involved in, or one that can be connected to a larger storyline continued later. Of course, some "random" encounters are just that, a chance meeting, and that is OK. Example: In the adventure N1: Against the Cult of the Reptile God (AD&D) I put in a random encounter of a recently attacked and destroyed merchant caravan, the PCs arrived about half-a-day too late. If they decided to go on to investigate one of the 20+ rumors I had previously doled out, they would be able to connect this encounter with what is happening in the Dim Forest. (If they bothered to remember, that is.) I like stuff like this, as it makes the world feel connected, more real somehow.
Some cool NPCs and special monster NPCs are nice. I like to give intelligent "monsters" like hobgoblins, orcs, etc... some character class levels to give them more depth and make them more challenging.
*It doesn't have to be a dungeon, perse, I like more realistic ruins such as ancient keeps which still have some above-ground stuff (the one from the MERP RPG is excellent as an example of this), or old settlements spread out over an area where the "rooms" are the old buildings in a variable state of (dis)repair.