YOURSELF!
Its the toughest battle I fight everyday!
There are TONS of excuses any one can muster but YOU are your own worst enemy.
SPACE- I have seen great works in small space. Just see Norm Charbonneau's old layout. Amazing layout in a basement space that you and I grew up in.
MONEY- More excuses. I have bought scale equipment at HO prices... just have to look for it...or practice sprucing up some semi scale equipment at bargain prices. You don't need that new engine in the catalog! H&LL you can buy 4 TMCC scale engines at that price.
NOT REALISTIC ENOUGH? Really stuck on that 3 rail are you? Then paint it black... otherwise you need practice on making some good scenes. As my Dad would say stop your cryin' , do something about it or move on! 
Reliability Problems?! Have you ever played with TYCO Night GLOW HO trains as a kid? Have you ever owned an older HO Brass engine? Now that was a lesson in frustration!!....
Lack of ability?!!!! - How many times as a kid did you break something and over and over again take it apart and eventually fix it? It was pretty triumphant when you fixed it wasn't it? Nope I am not using this excuse. If you have enough tenacity in this hobby there is NOTHING you cannot do. And there is PLENTY of help in this forum.
My biggest excuse is having too many distractions. But every night after I put the kids to bed. I head to the basement and either run trains or take that next BIG project on ... now lets see where is that bridge to no where going?
The secret is to just chisel away at it.
