What are the limitations of the game (number of rooms, number of objects, etc.)?
The limit is currently 32767 rooms, objects, etc. However you'd be unlikely to reach that before you encountered the 32767 lines of ASL code limit. It would be very difficult to get anywhere near that though!
The compiled help file is good, but I sometimes like a printed manual. Has anyone done a printable help file, tutorial, etc.?
Al a.k.a MaDbRiT has made PDF conversions of the help file in the past, though I'm not sure how much has to be done manually. I've toyed briefly with scripts which can convert the entire help file into one LaTeX document which can be converted to PDF. They work to some extent but they need a fair bit of tweaking.
How customizable is the gameplayer's perspective? For example, if my game is on a deserted island, will I have to use references to rooms, doors, etc? Or can by object references be more realistic? Hope that makes sense!
You can have Quest describe everything any way you like - you don't have to use built-in descriptions for anything. This means you can completely customise how things are referred to. Some people are even using this to write games which aren't in English.
Can I have more than one character in the game? Can I *play* more than one character in the game. Is this what the multi-user version is for?
One person could play multiple characters in a game - you just need to set up some mechanism for that. Perhaps you'd store in a variable which character was being played at a particular time, and have things in the game respond appropriately.
The multi-user version allows multiple people to play at the same time in the same game world. They can interact with the world and with each other.