So before - in that time when Alaron talked about the difference between Agility and Arpen - secondary stats varied all over the place. Agility was not as good as it is now (it only offered 1 AP), strength was fairly good, and the real debate for high-end druids was whether to go arpen-crazy or go agility. Arpen was an odd, nonlinear stat that gained value as you got more of it, which meant that having 1000 arpen was a very different value for arpen compared to having, say, 300.
This was all confusing and odd, but it meant one big thing: stat weighings really mattered.
But now? Now primary stats rule all, for everyone. The primary stat is the best stat by a long, long margin for all but the most confusing specs, and ferals aren't a shiny example. In this case, agility is 3 times better than the next secondary stat.
So there's never a question about what to gem for: it's agility. There's never a question about sockets, really - unless you're getting 3x the value of agility, it's always worth it to get agility. In practice this means any socket bonuses get ignored unless they give +20 agility or are a red/prismatic socket. So as far as your gear goes - it's all agility, all the time.
That means for the most part DPS classes have almost no flexibility in what they gear for. There aren't many options for DPSers. They almost always have to get 4p of the latest tier, then they get one offpiece, then usually have a choice of one set of boots, belt and bracers or maybe two if they're lucky. They have one weapon that's king, perhaps two. Trinkets are very rarely make it or break it and don't offer reforging ability or gems most of the time anyway. Rings, amulets and cloaks are also one or two per slot at best. Now those things offer some customization, but most of the customization is in reforging and swapping one or two pieces, maybe.
How much does that really matter? The answer is...not a lot.
Let's take a current 372-geared raider like Reesi. Here are their total secondary stats:
The total value for all secondary stats here is 5375. (for these purposes it doesn't matter that she reforged to dodge, since it's only the raw values we care about).
Now, let's take the preweighted values from wowhead for this:
Mastery : .35
Expertise : .29
Those are older weights from the 4.06 days, but for this experiment they'll suffice.
Let's do a thought experiment here: what if you could convert all secondary stats to the absolute best secondary stats possible. Let's say your gear was perfectly ideal (in this case haste/mastery on everything). We'll even split it 60/40 in favor of the best stat. What value do we get?
5375 * .60 * .35 + 5375 *.40 *.32 = 1816.75
That's the best we could possibly do.
So what's the worst? Well, we went for full hit/crit (and for this, we'll assume no hit capping). In that case, since they're the same value, you get the following:
5375*.6*.28 + 5375*.4*.28 = 1505.
So it's a huge difference, right? The optimal is 20% better than the suboptimal, and...
Oh, wait. That's just the value of the secondary stats. As far as a DPS gain, you can assume that one Hit/Exp correlates to about 1.0 DPS, which means that the difference here between the best possible value and the worst is...
So if you got the absolute perfect gear you would be 1k DPS better than someone with the worst possible gear choices. Which, given current DPS levels for most people at that gear level, is less than 5% difference.
This is why Alaron says things don't matter; it's not just that things are so close to each other that it really isn't a big deal one way or another. It's that realistically there's just not that much you can do with your gear. Your gems are spoken for you. Your gear doesn't have a lot of variety. As long as you're not going above hit/exp caps, stats aren't going to be wasted or horrible one way or another. This wasn't the case back in the day; gear choices and going full arpen could make a 20-30% difference in DPS. Now? not so much.
Note that this does not apply in one special case: Unheeded Warning. Thanks to the direct damage scaling increasing so much in the 4.2 patch this trinket is getting a large boost too; it is competitive with the heroic agility trinkets. Once again, ferals are going to likely be using a trinket that is from a prior tier well into the next one. Just don't use it on fights with tons of adds.
What this really means is that how well you do on a given fight is very much dependent on the fight itself, and not your gearing choices. If you do a lot of AoE, you'll find hit/exp/crit to be better than haste/mastery. If you do a lot of start & stop of fights (like Atramedes) mastery will be better. If you have a standup fight haste is going to be better. More berserks? Haste will shine. Lots of target swapping? Hit/exp will do better.
Ultimately this also suggests something else, which is that for progression fights where it really matters reforging should be done. It won't make a huge difference - the math above says that the upper bound of your difference can't be more than 5% no matter what - but when every little bit counts, that's a place to go. Understand what your role is and how to best do it.
Also note that if you care a lot about consistency, hit/expertise are going to win almost always. They are similar to stamina for bears in that while they're not optimal all the time they're never, ever wasted and make life easier. Given how close things are, you won't see much of a hit to your DPS and if you're not an amazing player chances are you'll see significant gains even on fights that don't punish misses.