Okay, I'll try to make a few points here in the few minutes I have. Please keep in mind that as general points these are generalizations and don't apply to ALL modules produced by necro.
(1) Despite our "meatgrinder" reputation we do presume some "role-playing" by your PCs. Good god man, if it is a scary blob of poop that you can't hurt, RUN AWAY! Adventurers should want to survive to the next day, or at least want to plan tactically. They should be smart enough to run away. Yes, in video games you can reload a saved game, players need to understand that this is pen and paper gaming. Role-playing can mean negotiating or it can simply mean playing your character in a manner that hints of a small sense of self-preservation.
(2) Do NOT "clear the level." The video game mentality just does not work for most Necro modules. Two of the toughest encounters in Hall of the Rainbow Mage are NOT necessary for "finishing" the adventure and even if they PCs successfully defeat those NPCs/monsters they could severely weaken themselves for later battles. The smart thing to do in those cases is retreat and try a different approach.
(3) Good game design means that sometimes we presume access to certain spells and actually plan encounters forcing you to use them. A huge pool of acid might be almost impossible for a lvl 1 party to bypass, but a lvl 9 party should be able to get past it easily. If your party does not have single-class casters or is low on casters or magic (or hordes it) they could make things much more difficult on themselves than needed. This might mean that if a party is written for 4-6 7th level characters and you decide to start your group at lvl 5 because they have 8 players and some NPCs, well, you'll have problems.
(4) NPCs and higher level intelligent monsters are presumed to be played intelligently. They also run away to slay the PCs another day.
(5) It is a game. In order for a game to be fun there has to be some risk based on the rolling of the dice. There is a thrill in knowing the DM has missed with all of his NPCs 2 rounds in a row and is due for a crit or solid hit any time now ... and you only have 8 hp left. Yes, no matter how well prepared you are, bad rolls can doom a party, that is, for me, part of the thrill.
Basically, we presume PCs are willing to run away and plan thoroughly before returning (meanwhile NPCs are planning as well), we try to force you to use your resources in new ways, and encounters generally demand careful, intelligent, and organized play from a group of characters.
A few random, disorganized thoughts from,
Patrick (one of those other Necro guys)