Interestingly, nobody mentioned the hand evaluation forums, which are one of the best ways to find and fix leaks in your game.
For example, ArkhamAsylum, for the last couple of months, you've just been spamming the introduction forums mostly with one word replies. I hope that you're not doing that only to be able to apply for staking or something similar. So if you are not using the features, they can't help you improve.
Troshacha, you don't seem to be too active on forums either.
Another valuable thing IMO are live coachings where you can easily work on fixing your game if you ask the right questions.
Now, to be objective, I do agree to an extent that the higher ranks don't benefit so much. You get most of the benefits as a bronze/silver member, whereas if you are gold/platinum, you don't gain many additional benefits. This is especially true if you grind a lot at micro stakes. Then, for example as a gold member, you have access to videos of like PLO400 whereas you're still playing PLO25 and the videos about PLO400 aren't that helpful. I do understand that this needs to be somewhat balanced as not everyone grinds a lot, but it is something that should be taken into consideration.
The black and diamond members have some obvious benefits in form of parties, forums and the top 250 promotion and can benefit quite a bit from rake races as well, unless they play on pokerstars of course - but even here it balances out as you probably pay less rake than on the small poker rooms, which means that you don't really need additional rake races.
So the group of players which might feel that they don't benefit enough are probably micro stakes players, maybe small stakes players who have (could have) gold/platinum statuses. These players might be able to benefit more by (as already mentioned) grinding till silver status and then playing on another poker room. So perhaps focusing on improving the features for this group of people would be beneficial for you.
Now in a way, you are already doing that. From a player's perspective, I can tell you that the "get $100 extra if you reach platinum this month, get $50 extra if you reach gold, etc." kind of promotions are very motivating, whereas the "get free platinum status if you earn 50SP each day for 20 days" kind of promotion isn't as rewarding and motivating for this specific group of players, as they don't see many benefits in being platinum compared to gold.
I hope that that was some constructive feedback :).
P.S. I do agree with one more thing, you should really look into using your mailing system to send out some surveys. I actually find it quite surprising that you don't use that yet, as it is a great way of getting into your customers' heads and giving them what they want.
-SF