I’m playing a level 45 Outlaw Rogue with an ilvl of about 46, currently working my way through Warlords of Draenor. Suddenly, I began to one-shot mobs and not really take any damage. My Sinister Strike alone hits for 8k-10k. I solo’ed Tarlna. At level 45.
I mean, I’m not complaining (leveling a rogue is a miserable experience), but this might not be a desirable state in the long run.
Also, before this, I sometimes didn’t appear to do any damage to mobs at all, making progression in some expansions impossible.
Just letting you know something weird is happening during the leveling process.
You need to enable chromie time for mobs to scale up to level 50, once you reach that you’re supposed to go to Shadowlands if you have the expansion, or it’s your max level otherwise.
Chronormi is right on the mark here - each expansion area natively only scales to a certain degree (outside Chromie-time), and after going past the associated maximum character level you will indeed massively overpower any creature found there. 
Yeah, it’s just that I kept getting full xp rewards for quests in that area. However, that only lasted until I leveled, and everything seems normal again, now. Thanks for the responses!