I'll chime in here since I play a barb, and am relatively conscious of weighing mechanical advantages. Disclaimer: I've not yet delved upper-teen lvl content beyond Curst.
TL;DR As it currently stands, spending all your feats as a barbarian on rage feats is a mechanical waste.
No, really! The most potent use you get out of it is the SR, which can be approximated by the Greater SR potion variety, or a cleric or druid at your back. A lvl 16 cleric or druid can give you exactly the same thing for much longer. If someone says that the barbarian rage/SR feats are still useful for soloing or when a cleric/druid isn't necessary, then they're looking at dungeons below their barbarian's level for xp, where SR doesn't really matter in the end. If you want SR, you want it to last a long while. If there's a high-level barb out there who thinks differently, please speak up.
Perhaps even more importantly, there is an enormous amount of utility in the vanilla feats. Weapon focus, Improved Critical, Blind Fight, IKD, Expertise/Improved Expertise... these are all more attractive than the barbarian feats, barring Stone Rage and maybe the two Extended/Increased # of Rage feats.
As it stands, you either ignore the barb feats altogether because they detract too much from mechanical efficacy, or you take them all because you love getting SR while raging. A few take only Stone Rage because they're nabbing every bit of AC possible.
I love BattleCupcake's suggestion of adding feats that improved upon their unique qualities, like DR and speed. Stuff like that would outweigh the utility of Expertise/Improved Expertise, for instance. Not for all builds, but some, at the very least. Note, while the 3/- DR isn't negligible--as Pav stated--this "unique" aspect of the barbarian can be replicated by enchanted adamantine armor, or so I've heard. Making it possible to outshine an adamantine item if you invest feat(s) sounds like a really good way to make it truly unique to barbarians. Even only adding 1/- DR by feat, ideally locked behind lvl 17 barbarian, would be a very nice pat on the back for resisting the temptation to dip into rogue and/or fighter. (Edit: suggestion retracted per the reasoning below in a later post.)
I understand the argument that RP, the character, should be what makes them unique, but mechanics--especially all these interesting feats on POTM--are meant to underscore those differences. Why not add or replace a few to make that underscoring work better?
All that said, this is a relatively minor issue compared to stuff like re-designing crafting systems. I'll not speak too much on it, if only because it's a footnote on the list of important issues for the devs to deal with.