Yeah, I’m not saying Arms is bad for tanking while leveling or anything. I’m sure it’s adequate, and maybe even better at lower levels. But above level 40 and before endgame raiding w/ stacked buffs and consumables (which scale much better with raw damage talents), I think Prot is the most effective tanking spec. And I think one of the reasons is precisely because it doesn’t scale well. The flipside of that is that it doesn’t require a lot of stats to be effective.
Also, Prot allows you to take tactical mastery (which Fury/Prot does not allow) and with that and other talents in the Prot tree, it gives you a lot more tools in your kit for controlling packs of mobs (which is pretty much everything in dungeons, bosses are easy). Cheaper abilities, a lower cooldown on taunt, a stun on demand, the ability to charge, zerk rage, intercept, mocking blow, etc. and keep all your rage.
If was just hitting or nearing 60 and wanted to tank a bunch of dungeons, I would advise speccing Prot instead of Arms or Fury/Prot, because I think it makes life easier at that gear level. It’s better at controlling trash packs and easier on your healers because it doesn’t require that you dual wield or use a 2-hander to avoid being rage starved.
You don’t want 5 points in TM while leveling. Sure, you miss out on Anger Management but all your skills only take 15 or less rage anyway, so those extra talents aren’t accomplishing all that much pre-whirlwind, and you’ll have other priorities for those precious talents once you do. This allows you to put points into impale, Imp overpower, imp charge, and still have adequate TM at level 26, as such:
The whole point of this is that you’ve hit your core improvements for hitting stuff with a big 2hander, which will really help you solo, and the damage you do will make wake up for what little you lose from not having defiance(15% more threat from the garbage threat you produce at low levels is still garbage threat).
Also, before this point, you’ll be so rage starved that you’ll take your shield or pants off just to take more damage and get rage anyway. After it, you’ll want to actually start putting points into the core Prot talents (Shield spec, dealers choice on tier 2 while leveling, then into defiance) so that your extra threat gen starts to come through right as your allies are starting to unlock their own big talents. Thus, level 41 might look like this:
After this, you’ll want to finish out to sweeping strikes in arms as dungeons have more enemies and your allies can no longer survive pulling aggro easily, then improved taunt and sunder for better options in the chaos. At around 52 is the time to hit up your trainer and do a complete rebuild with an eye towards your expected tanking style of fury tank, or deep prot, and go from there.