See my reply to Lexkalin above, but tl;dr it's fine for me, but might be restricted if you're OCing or have a more powerful GPU than a 1060.
Yeah, a Define C with its side intake probably isn't the best for a 1080, even though it is a 1" clearance. But I've got a 1060 and have no plans to OC it, and haven't had problems with heat while gaming. Most of the time when I need power its for things like Handbrake, and while the Hyper 212 EVO + rear fan is fine for that, I recently upgraded the CPU cooler to a MasterLiquid Lite 120 (because it was on sale for $15 AR, can't go wrong with that).
But other than what you mentioned, the Define C is basically perfect for everything else.
I like RAM. And I tend to do a lot of virtualization stuff.
http://www.stickeryou.com/products/clear-stickers/491 - When I ordered it was about $32 for 2 sheets of kiss cut clear stickers, and I was able to fit 6 of those stickers on a sheet. Not cheap, but the quality is good. I've got one on the back of my car window, and it hasn't degraded.
Looks good! But I think you need to invest in a desk. Playing on the ground must get uncomfortable.
Yeah, it's not ideal but it's the only way a clip-mounted AM2/3/4 cooler can be mounted. Once I get a Hyper 212 EVO and the AM4 backplate mount, that'll be mounted horizontally. That being said, I'm not OCing anything and it's been perfectly stable under load, so I haven't been worried.