Well now after creating 3 successful modules,
I can definitively say what is needed.
1. At least 1 more tileset and more placables. After making 3 different modules and doing as best as I could to make each one appear different, I will admit the Crypt tileset is very versatile. I can make either a Crypt or a Dungeon adventure with it. However after 3 it does start to run out of different permutations possible. And without more, what you can create starts to thin out. Different lighting might help a little but still you will come to an end sooner or later. I would say the best option is a caverns style tileset to accompany it. Also things like staircases and general dungeon placables for monsters in the dungeon.
2. At least one type of outdoor starter set. Even if you aren't ready to release an NWN2 style outdoor environment maker... that's OK.. BUT... one of the problems is starting the PCs off on an adventure. Most often their characters will have default sets of spells or no appropriate gear because you cannot stop at the merchants prior to entering the dungeon or rest at an Inn to regain spells. So they enter lacking even the simple things like Thieves tools, Plate Armor, Shields, weapons more appropriate to their builds etc.
You (Toolset Devs) don't have to give a City Tileset right now... all you'd have to do... is cut out one of your own Inns and slap one of your own pre-created merchants in a single building as a generic "Pre fab" per se... for all the creators to use.as a starting point for an adventure. We can place the starting point there and link it to our individual dungeons. We can then add a chest of goodies specific to the adventure or appropriate magic items for levels... Win win for everyone. Later you could replace this with an actual City Tileset. It wouldn't be too much work... and it would solve a great many issues builders currently have.
3. The ability to manipulate Monster stats and names and/or unlocking higher CR monsters. While building Ruined Keep of Agramar I really needed those orc leaders and berserkers, I could make do with Ogres and such, but it does help to have the full run. In Crypt of the Wight King I really needed a monster for a Named NPC who was just Raised by the Necromancer, but could not change the name of the Monster to make him identifiable, and a single wight in a whole field of wights isn't going to exactly stick out. Also it would have helped to be able to change the Necromancers name to something other than "Necromancer".
4. Definitely fix those entry/exits to and from modules corruption issues. There are a lot of people who would like to link multiple floors/modules togather but cannot currently because of the corruption risk destroying all modules involved.
These I would say are probably the critical mass issues.
Not-specifically-related-to-Dungeon maker-Issues:
1. Players can't export their characters so they don't get Experience points nor treasures gained from them. While not critical, it is kind of a downer for them not to have anything to show for going through a quick dungeon adventure. And they can't start a character at say level 1, go through someone's adventure to level 3, then go through someone else's level 3 adventure to level 5 and then go through someone else's adventure to level 7, etc etc...
Wishlist:
The ability to create outdoor areas. Yeah I know this will be a ways out and I know how complicated building an outdoor area really is so I don't expect it for a while, but it will add a great deal. Creating the actual area you enter the dungeon at where you find the cavern or ruins or fortress really does bring the adventure home to a player.
EDIT: OH there is one more thing... getting on that bug that causes off and on EXP gain in dungeons. Sometimes a character will not gain EXP in a module until a a certain ways in, almost like it freezes briefly, then about halfway through they will start gaining EXP. This will also need to be fixed, doesn't affect it right now but could come into play once people are able to link multiple modules togather.