I believe what @lamefun is referring to, especially for older versions, although this is an overall issue we still need to address in current development, is the unusually difficult learning requirements trem has had on so many levels for just doing the basics, as well as the unusually difficult usability. The way things have been for trem, you pretty much have to be an expert in trem just to be ok at the game play, to be able to host a server, to be able to do even basic modifications to the code, just to make an ok map, just to be able to build the game, to be able to have good settings, to be able to troubleshoot common issues, even to be able to just get and install the game (although installtion of the 1.3 alpha client is way more straightforward than it has been for older versions).
It is one thing if to do advanced things you have to get more into an expert level, but if you have to be an expert and fully dedicated to the game just to get started, that is a serious problem. There are so many aspects that are unfriendly and not intuitive towards new players/modders/server owners/mappers. Such extreme levels of difficulty for entree is completely unnecessary, and it gets in the way of growth and development of the community and of the game. That is completely unnecessary, and those issues can be addressed while still having the game and its development offering intellectual stimulation and challenges.
I think very much related to this is a video that @lamefun posted awhile ago about the difference between challenging vs punishing.