If you are 'ok' with pvp gear then you'll only getter better with pve gear. Lets take a pair of random ele shaman leggings for example.

A typical pvp leg would have 400 resilience and 200 crit, the same ilevel leg in pve would have 200 haste and 200 mastery (besides the obvious int and stamina). Resilience is totally useless in pve, so basically you near double your offensive stats by swapping out pvp gear for pve gear.

There are exceptions though, some classes really favor 1 stat much more than any other stat. So as ele shaman you might be better with a epic pvp bracer than with a 333 pve bracer, just because the former has way more INT on it.

What you should do is read up on your class and check which stats are mandatory. Whenever you get a new item it's pretty straightforward to compare if it's an upgrade or not (take reforging bad stats into comparison as well)