Protecting yourself from briars / brambles / jaggers while mountain biking
I have used shin guards since the 1980s, growing up in Hawaii and now riding in Utah (both get pretty hot) and I found that wearing knee high socks help with the "heat" factor, but even then, as long as I am wearing shorts, I don't even think about my legs being warm. 

I have been using the LizardSkin shinguards for about 15 years now and will continue to use them as long as I ride! (I am still riding the same pair, they are frekn very durable!) 

For the shin-whacks nothing protects better, they have a V-shaped insert that virtually eliminates any hit. And if you want to ride without that layer of protection (or to wash it) you can take that insert out really easily. Even without the insert, it has a pretty durable front with some padding that would help with any sticks or whatever slapping you. 

The neoprene has holes for ventilation but not really a big help, but the back gives me a gap of about 2 inches, and I have skinny legs. I have ridden a lot of downhill trails and they have helped a lot through the brush. 

They even come down low enough to give your ankles some protection.