At only level 40, gear for a Prot Pally really doesn't exist yet. Sure there are a few pieces with +DEF on them, but not that many. Until you get the higher talents, stamina doesn't convert to spell power yet either. The best way to generate threat at level 40 is probably going to be +Strength and +Agility. You still need Stamina to survive, so don't ignore it.
Once you get Touched by the Light talent, stack stamina like crazy and it will greatly increase both your surviveability and your threat generation. You will also still want strength (which was not the case for a Prot Pally in the past) once you have Touched by the Light, but it is still by far not as important as stamina once you have the Touched talent.
For now, any plate that is "of the Bear" is certainly suitable, and in your late 50's to early 60's, any "of the Beast" plate is great. Once you hit mid-60s or so you will start to put together a more traditional tanking set of defensive/stamina gear.
For now, if you get a good piece of armor or a shield with +def on it, don't ignore it, but if a better piece drops that doesn't have +def, use the better piece.