Circling the peninsula the dark portal is on is great, and what I used on day one (cause I knew there'd be spawns there nobody would check) to ensure I got myself up to 325 for adamantite. My mining is now up to 375, but I jump-started it to 330-something around the portal until I got bored and wanted to actually do stuff on opening day.
What makes it especially frustrating is the utter lack of mining nodes in Zangarmarsh. There's effectively nothing in the middle of the zone, so you have to generally rely on Hellfire Peninsula, which is overcrowded for the time being. Eventually it will thin out, but until then, circle the portal or run the perimeter of Zangarmarsh. You start getting adamantite in Terokkar, so it's no guarantee on fel iron there.
That said, I'm a jewelcrafter. I don't care what metal I have as long as I can prospect it. If you are an engineer or blacksmith, I am your bane. I am a parasite that simply mines and "disenchants" the ore, regardless of what it is or how much of it I get. So it doesn't matter to me how much Fel Iron I get so long as I get my mining past 325, because I could level up completely on my prospecting of adamantite ore if I can prospect it. (Granted I need 325 skill to do that, but there are other sources of gems.)
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Only the exceptions can be exceptional.