The hardest thing (and I can tell you it was hard work for an entire year) is the coding part. It's not only coding in the Module System, but the website, and linking everything together taking in account security. There is thousands of lines of code in only PHP, and thousands in Module System, it's many work and there is still many new ideas and things in mind to add in patchs.
If do you think the item set is the main thing that matters on a mod I respect your opinion but I don't agree. Nevertheless, we are planning to add much more items (we already have our own items and other OSP items), but it's hard because we have to balance those weapons, set a price or a craft recipe (if it's for crafting), etc. So it's a complex task to add things, since this is something like a MMO RPG, where everything needs to be as balanced as possible.
The gamemodes the hardest part? Well, the entire mod was the hardest part. I mean, the website, the connection between website and the mod, the gamemode, the classes balance, all the items with prices, all the huge crafting each one with his own recipe, all the systems of the mod (private shops, auction hall, private trading, 7 different factions, all the special items (like potions), waves, troops design, inventory, admin panel, etc, etc).
I think some people underestimate the complexity of the mod. I would suggest them to read the "CSR Bible" (which is not available at the moment because CSR is closed yet), play the mod with all the gamemodes for a time and then come back with their opinions.
I think it's unfair to judge the mod without knowing it. I mean, even on this thread it's not full explained properly, that's why we'll take back the CSR forums where everything is explained properly and that's why we'll be releasing CSR with a video in the end of september. On that video all the mod will be explained in detail, so people don't have to look for information any further.
Thanks for your comment.