There are plenty of things that are easier to design than to implement.
There are plenty of features that even harder to design properly, so they will be fun, not just complex and boring.
About the question - I often cut potentially funny features if their implementation seems too difficult. The problem is that I often don't look at their ideas. I'm sure that creating interesting multiplayer platformer with procedurally generated levels (such as described JumperRL) is very difficult, so such idea won't come in my head.
I'm sure that creating interesting MMORPG (roguelike or not) is just impossible, so i won't even try.
I won't try to create roguelike with detailed battle system, complex body parts and material system, just because I'm sure it won't be fun without some "spark" in design.
Well, even creating something like ADOM or Angband, creating pure roguelike without "twist" seems impossible to me, because such roguelike will be boring. So in my projects I always use an additional ideas, e.g. unusual setting or battle mechanic - I'm not sure that i can create something interesting without them.