I can answer this for you. Attack and defense definitely do matter in the Taken King, as that is how we are rated for combat instead of light level. The new light system takes the average of your attack and defense, and certain activities will check this light number to ensure readiness. So you will likely not be able to wear a set of blues, or greens to do the new end level PvE (I could be wrong though, because VoG was originally done by most in almost all blues).

The armor does not determine your level. But it determines your defense stat, so the better gear you have, the better defense stat you have, the more damage you can take. So yes you can be lvl 40 with any kind of gear but at a cost if wearing low level gear. Hope this helped!

I personally don't agree with having a green piece of armour having the same defense value as an exotic per say but for your new light level to he increased it is an average of your stats on all equipped items (weapons attack rating, armours defense rating) and also something to so with your class items ghost and artefact. all that will make your light higher as in a video on YouTube there was a character who was level 40. had 216 light level and had thorn and 2 legendary weapons equipped. a rare ghost and all rare armour with an artefact. apparently the highest light level is over 400 now