iPads have air gaps that can cause eye strain.
I returned my iPad mini because I was experiencing eye strain when reading off of it. I later found out that it was caused by the air gap between the lcd and glass. The 4th gen iPad also has this gap. Basically, when you look at an iPad, you're seeing two slightly different images. This makes it hard for your eyes to focus.
If you want to feel the difference, just read off a glossy display without an air gap and compare it to reading off of an iPad. You can tell if there's an air gap by turning off the display and looking at the reflection. If you see two slightly different reflections creating a blur, that means there's an air gap. If there's one clear reflection, that means there isn't.
Don't buy an iPad until Apple uses a laminated display on them. An air gap isn't something you should have to put up with, especially for what they're charging. Even the Nook HDs, Kindle Fires, and Nexus 7 all have laminated displays.