What do you think constitutes as a good para (reply) and what do you think is an example of a bad para (reply)?
A good para reply strives to match the length of your partner's. It may not be a spot-on match (slightly less or a bit more) but if someone writes a solid four paragraphs, a good reply should be between 3 - 6 substantial paragraphs in return.
Any para reply should provide more than a bit of action and dialogue. A good reply should explore your character's current state of mind, their physical condition, and connect it to their interaction, the environment, and their dialogue. A good para reply also maintains any previous condition your character may be in. If your character has a cold, reply 1 - 7 should all reflect that your character has a cold in some way, so that continuity isn't disrupted or lost. A character using crutches to get around in one reply should not suddenly be sprinting up the stairs for something in the next.
Another aspect of a good para reply is that it continues where the last reply left off. Though it's just fine to include your character's reactions in relation to something that occurred earlier in your partner's reply, the bulk of your para should be devoted to moving the thread forward, progressing the plot central to that particular thread.
I personally find a para reply that isn't three strong paragraphs or longer to be a poor one. I think many roleplayers, especially when first starting out, feel that when it takes them a long time to write out the reply involving thoughts, actions, and dialogue, that it's too long within the time-frame of your character's interaction, so the urge comes in to make it shorter. But you have to remind yourself (as I often do) that though it may take you anywhere from 10 minutes to an hour (depending on your level of focus or connection to the particular scene) to write, the actual time it takes your character to think, say, and do things reads and is imagined to be moving much more quickly.
A bad para is one that doesn't give your partner enough to work with in their reply, whether it's from lack of length or lack of substance. If you go into a para with a partner without a rough idea of where you both want to see the scene go (such as open paras that get picked up at random) then perhaps you should get in touch and discuss where you can mold this into something substantial. Perhaps you've run out of ideas; getting your paraing partner's thoughts and opinions can foster some new ideas, possibly even for not only the current para but future ones.
If your partner's character has several moments of dialogue, understand all these instances of dialogue to happen before your character can make a rejoinder. If you write your character to speak back to every piece of dialogue, it can become convoluted and read more like three paragraphs taken out of three different para threads. If that doesn't make much sense, ask for clarification and I'll see what I can do. I've had people write this way with me and it's extremely limiting and difficult.
Lastly, a bad para reply is one that parrots the thoughts of the other character in a different manner or seemingly responding to the other character's thoughts, as if they are either 1) thinking the same thoughts or 2) your character is somehow privy to the narrated thoughts of the other character they are interacting with, which--unless your character is telepathic--isn't the case. This is something I've just recently encountered myself and it does just as much good for the progression of a para as the interjected responses to dialogue.
In the end, use your best judgement when you write and always think of all the aspects of your character while you write--what are they thinking? How do they feel? What do they want? The bottom line is that a good para reply is loaded with the little details that make good writing while a bad reply is lacking in one or more areas.