I want to throw a new wrench into this given what I said before. I stuck more on theme with the types/genres that screw me up, some even by style of love/hate (T/SRPG stuff for one.)
How about this? My biggest by far -- motivation. Mine is greatly shot many days a week. It may be that I'm feeling lazy, I'm tired, other stuff gets to me, or I have to share time with my tv recordings, manga, old books, etc. Motivation trumps all of my gaming issues, the true heel. I'll think about it during the day, think to myself I will throw X into my hand and play it, and subconsciously or consciously I'll conveniently "forget" and then not do the gaming or gets late enough I won't have the time to get the value out of it so I won't.
That's my biggest most obvious answer to the question right there, motivation. If I could just stop that and actually turn something on I'm good to go for 30min or 3hours just like years ago. Now I could go an easy week letting it all get dusty.