As modders, what is the most limiting factor that you face from using one of the current generation of engines?  i.e. contention creation tools, scripting language, engine aspects not exposed, documentation, support, etc?

I would have to say API documentation is probably a big one.

It's ok to sit and figure out some stuff by the examples in the written code, but sometimes it nice to be able to just print out a nice piece of documentation with the calls spelled out.

I think it's a mixture of what you listed

I think one of the most limiting factors for mod teams is time.  

One of the advantages we had while working on 'Midget Wars' (beta 1) was that we worked on it full time for ten weeks - as part to the 'Dare to be Digital' competition (http://www.interactivetayside.com/dare), but now I'm back at university doing my senior honours year we've only been able to release one small update.

Another limiting factor can be peoples attitudes (I'm fairly sure this doesn't apply to anyone here) - members of the team that cannot be bothered to put the effort in, learn something new.

I think people who act like this are more of a limiting factor than lack of documentation or such, as anyone who puts the effort in can learn to work with anything, despite lack of documentation or access to engine details.  A personal example is that none of the 5 of us who made Midget Wars had never worked with Q3 before, but by a couple of weeks in we were fluent in the workings of the Q3 code.

I think all the reasons mentioned in this thread can have an effect.  For example lack of documentation can make people reluctant to learn to use the code/tools.

The memory is a problem. How big can a map be, so it runs fine under a certain engine. The other thing: it needs to fit in eachother, it needs to be solid. You have to have a good and solid idea, so it runs.
Concerning other things: you can cut in something. Focus on maps for instance, and leave skinning and moddeling. Wenn you make a singleplayer game, then you could leave coding.

The other thing is: multiplayer involves sometimes more (!) than singleplayer. It isn't therefore better or anything.

You can focus on really the game you make, the content.
What besides that do you need? For instance just a piece of coding. Add a thing new.

PS: A mod needs to be finished too.

I have another thing to add, since it's sort of been mentioned.
Map sizes! I think map sizes are currently way too small, the only engines that I know of with decent map sizes are Torque(Tribes 2) and Serious Sam.

I kind of disagree with that. If the map is too big, the fun factor imho disappears when you have to walk for 2 mins, get shot and walk all over again...