The general concept is that obstructions to a quick evacuation should be minimized during takeoff and landing because those are the most likely times for an evacuation to occur. Reclined seats, open tray tables, curtains, loose luggage, etc., all make it harder to get out. (Your reclined seat may make it easier for you to get out, but not the luckless person behind you.) Also if there's an abrupt stop, you don't want to get bisected by your tray table.

Window shades are supposed to be open so the conditions outside can be evaluated before an evacuation.

The deal with the electronics is a can of worms that's been debated here lots of times. The main issue is that the possibility of interference hasn't been 100% ruled out, and take-off/landing are the least forgiving of the flight phases. Nobody wants to say "go ahead and use them" and then have a crash.