Thanks for the very constructive replies so far, it's very helpful to see
why you use one feature or the other, and how the programs make a difference.
I've noticed some users find the orange stacks to be intimidating, but it wasn't meant so, maybe "expired" is not the best choice of words. I always thought they are cards "ready to be reviewed". The urgency of review varies from stack to stack, but no program in the world can read your mind, it's a best guess from an algorithm which is more or less skilled at deducting the optimal remembering interval for you.
I've been pretty stubborn with the red stack. I reckon there should be an easier way to review from the red pile, but as I thought about it again lately I started wondering if the SuperMemo approach may not be the best approach for material such as RtK.
Lately I reflected on the fact that if a SRS tries to be too smart, it wants to find the best intervals for you. But are those intervals really based on your memory's "capacity", or are they based on what best use you could make of your memory at the time of learning with the information at hand and the skills you built so far? To give a simple example, if the interval is set in stone to 6 days and you didn't pay attention while learning, you fail. But a SuperMemo-like SRS will say, ok let's review this in two days instead based on your rating of the answer. The result is that the program doesn't tell you the ugly truth : you didn't pay attention while learning.
So when you stop, what happens? Did you really make good stories? That's one reason I kept with the red stack and probably will keep doing so. The red stack is there not to tell you you've failed, because nothing stops you from clicking "Learned" and drilling again after all; it's there to tell you that you should probably look at the story/mnemonic and see if that's not the culprit, because this RtK method heavily relies on mnemonics/stories. This is possible because the site started as a review site purely for RtK, but I wonder if this should not apply to many other materials.
A learning program is not primarily entertainment, if it can make learning fun that's great, but that's also a regular reflection of mine, the subject of .. can you really make a learning game? (but I disgress and this would make another very interesting topic

)
And so I think pleasing the user is not the number one priority, it's better to be blunt and say "could you improve this?" rather than give the learner the feeling everything is alright and let them continue on shaky foundations. So that's one area where RevTK takes a stance, so to speak.
PS: my reflections above concern RtK material. Once you move onto vocab and material that does not rely heavily on mnemonics I think smart SRS'es like like Mnemosyne, Anki, SuperMemo etc are very well adapted. For the vocab area on this site I've already changed the functioning of the red stack, it's still red, but you can review it straight away.