Hey =) Nice video. Here's my $.02:
#1: Don't excuse your voice, sure we don't all like our voices, but yours has character.
#2: Its 10 minutes! thats not long for most such videos, they rant on and on for up to 30 minutes :d
I've never come to the point of making spawners in my hardcore worlds because I enjoy risky jumps so these might not be applicable:
#3: You could put your relays below the turtles and and have the turtles do a turtle.dropDown() instead? That way the turtle won't have downtime attacking, and would decrease chances of any skeletons falling down. If you make the last drop 24+ high the skellies will die anyway I believe.
#4: If you make the walls of glass the spiders won't be able to climb up.
#5: Make a quicklink to the time when you start explaining forestry so that anyone wanting to see just that can directly jump there. You could also just make two shorter videos.
Thanks again