EDIT: Oh dang this is huge. You're gonna want to download it >.<
I know a lot of my most recent submissions have been redlines, buuuut this is another one of those redlines where I do some nifty little demonstrations about things other than anatomy (heck, this particular redline is more about expressions than anatomy) and thought it might be worth sharing.
First off, the original drawings are ADORABLE. I tried to keep my redline overall pretty close to your style, though I definitely took some liberties.
There were very few blatant anatomical errors, so I won't say much about them. Just make sure that you're not bending anything into an unnatural position, and when in doubt, draw a simplified skeleton over what you've drawn to see if it makes sense. Also, try not to draw the ears on the side of the head when they're being portrayed as folded back. I suppose this is more a personal preference thing, but I find that the ears look much more expressive when they're drawn angled back; they just kind of look limp when you draw them on the side of the head.
Now I'll address some specific expressions! I'll start with the fearful one on the the top left (of the original drawings). This is one of those expressions where I think the ears being folded is really important. I drew them looking somewhat bent and contorted, which helps enhance the tension and anguish in the cat's face. Also, be sure to make her pupils small! Pupils contract to show fear. Don't be afraid to use the fur as an expression tool as well! Make her hair stand on end, and make it look a bit disheveled.
Moving over one face to the right (the unamused looking one), the main issue is the angle of the face. I can tell from the upper half of the face that she's looking down a bit, but the lower half of the face still looks like it's facing front. Draw the muzzle tilted down a bit. I drew one of her ears tilted back in annoyance as well.
The distressed face to the right of that has the same angle issue. Make sure her whole face is tilted downward. Overall though, this one's pretty good! I like how you made her "hair" droop a bit to match her expression. :3
Swinging over to the "know-it-all" sort of expression on the left side of the next row, we have another angle issue, but not because of consistency (I actually think you did the downward facing muzzle pretty nicely in this one) but because the angle doesn't really match the expression. I interpreted it as a sort of arrogant expression, so she should be turning her nose up at whoever she's talking to in an act of extreme confidence. The ears would likely be a bit perkier too.
The crying face in the middle is the main expression I have an issue with; it almost looks like she's yawning. The problem is that her face is too relaxed. Her face should be wrinkled, especially around the nose and between the eyes, to show anguish. Dramatically folded ears help too. Also, don't just draw the tears as some random gravity-defying blobs (they're also flying in all different directions); draw them streaming down the face, starting at the corners of the eyes. It looks a bit less dramatic, sure, but (I think at least) it's less distracting from her face and better portrays the expression.
Now to the smiling one on the right... fkdsjf I love this one!
! I really didn't change much on it; just the angle of the paw (it looks more natural and relaxed now) and the hair (made it a bit perkier to match the expression). Nice job!
Moving down to the next row, I didn't change much on the disgusted, "wtf" sort of face either. Nice angle! I'd just recommend folding the ears and maybe wrinkling the nose a little.
The face to the right of that had some angle consistency problems; make sure that muzzle's facing the same way as the rest of the face! Also, this one's got a major case of side-ears. Regardless, I think you portrayed the expression pretty well!
I didn't do anything to the straight-on smiley face, so now onto the little guy curled up in the bottom corner. All I can really say about this one (aside from the ears) is that there should probably be a few wrinkles between the eyes to show distress. That's all I can see though. Another really good one! ^.^
Oh, and on a more general note, those little spots above the eyes can be a pretty useful expression tool, as I noticed you didn't give the character eyebrows. Remember that as the underlying muscles in the face move, so will the skin/fur/markings above them. This means that those eye spots will move depending on the expression, and can be used to further intensify the emotion you're trying to portray.
That's it for the specific expressions. Now onto some more general advice.
You don't seem to have much trouble with character consistency; it was pretty easy to tell that all of the faces belonged to the same character. Still, it couldn't hurt to mention character consistency. It's important to be aware of the defining characteristics of your character. This will help you draw your character in any pose and from any angle while still making it obvious who it is. I drew a little example on the right that may or may not be totally accurate for your character, but it gets the point across.
Aaaaand that brings me to my final subject: character variety. Judging by the other character you drew in the lower right corner, it looks like you might be a bit shy when it comes to this (I could be wrong though). The random little dudes I drew on the right are there to show just how different you can make your characters look from each other, even when they're all the same species (all four of them are semi-longhaired domestic cats). Don't be afraid to play with all sorts of facial (and body) features; eye shape/size, ear shape/size, nose shape/size, the proportions of the muzzle, the features' overall roundness/sharpness, etc. Play around and see just how far you can push the features while still keeping the species recognizable.
That's about it I guess. Let me know if you have any questions or criticisms! I've been working on this on and off over the past few days, so things may be a bit inconsistent/hard to follow. Let me know if this is the case.
"Lackadaisy Expressions" I noticed someone already linked this to you in the comments, buuuut I really really like it and felt like sharing it again XD Check out how the artist plays with the characters features while still making them all look distinctively feline: [link]
This is a tad human specific, but near the middle of this section of this (awesome) tutorial, the artist goes into describing various expressions using arrows, etc. Check it out!: [link]