First thing what people don't notice is that it is sometimes possible to go somewhere where you can't see any exits - not even with search command. For example somewhere in room description there is mirror in room which seems to be like other world, you can search and not find any exits except that one which you came from. Most people just go out of room and think it was that but clever ones read description and see that mirror - and they try to go 'north' although there is no exit since mirror was on northern wall and they find new part of zone, totally hidden from people who only look exits and don't bother to read where they are. Very often used tricks in Mud zones are bookshelves, chests and cupboards in descriptions but not making them "visible objects". You need to read description and see that there is really chest and then try to open it. Too many mud players don't understand that room desctiptions are main part of rooms and they look only special objects like mobs, items on ground or chest which can be seen without description and if they do not find anything they think there is nothing. That is totally wrong, there can be many important things in room although you can see them right now, use brains and think - there might be something.
There is also trigger-puzzles which you need to do before you can continue to second part of zone. This kind of puzzles can be found in many zones and these require little thinking also and not only mindless killing of everything. Trigger puzzles are events which happen only after you do something special, like talk to mob and it tells you something after that or you need to press statues eye to open secret door. Sometimes trigger puzzles are very weird and hard to notice but mostly solutions can be found relatively easily somewhere in zone if you just keep searching and reading descriptions.
Keys are easy to understand for all people but infact keys are only small part of trigger puzzles. You need to unlock door to get trigger event which makes door to exit which can be opened. Mostly keys are easy to notice and simple to use but there is something between normal keys and true trigger events - special keys. Some items are keys although you do not know that for sure before you actually use it, or you do not even know it can used like that before something just happens accidentally when you have that special key. Special keys can be for example amulet which you need to hold to open altar of ancient god, look at item and maybe you can see picture of ancient worshipper holding amulet and secret door near altar is opening. There is many variations of special keys and I don't want to ruin your possible discoveries by telling about them before you see them, just remember that not everything is what it looks like.
Making logical list of different features in mud-zones is hard but all these simply small program codes which are copied again and again while only changing little they way these things look to players. I think these were the few main parts which can be seen in mud zones but in overall there is probably only two things - triggered events and descriptions - all other are just variations of these two codes.