The biggest challenge of pantomime is clarity. It isn't too hard for an audience to understand that a character is lifting a box without the use of dialogue. It is a bit more challenging to have them understand why the character is lifting the box and what emotions it is feeling while doing it. If a viewer can't understand what is going on in the character's head in the first viewing, then the animation is not quite working yet.
Great posing is the biggest component in giving a shot clarity. A good pose will immediately communicate what is happening to the viewer. A great pose will do it in an entertaining way. Entertaining doesn't necessarily mean that it has to be cartoony or insanely over the top. It does mean that the pose should be dynamic and interesting to look at. Comic books are a great resource for seeing clear, dynamic poses... and yes, I am a super dork. A comic book artist has to communicate both the external actions and internal motivations of a character in a single drawing. So basically, if your key poses could work well as a comic book panel then you are in good shape. If not, then you might need to tweak them some more.
A good technique for judging how clear your poses are is to look at them in silhouette. An audience can read the silhouette of a character much faster then they can overlapping body parts. Any actions that take place inside the silhouette have a much higher chance of becoming muddy and unclear.
Pantomime shots have a huge amount of freedom in them because of the lack of dialogue. It is a blank canvas. However, this can also be a challenge because you no longer have that audio to anchor your timing to. Sometimes creative restrictions can actually be helpful. The challenge in pantomime is making sure that your character doesn't feel like they are going through a set of meaningless actions. Try to imagine that there is an inner monologue going on so that it feels like they are thinking and then acting. The pacing of your shot can determine whether your character is simply doing some things, or doing some things in reaction to what is happening around them.
Acting choices are another huge factor in pantomime. I think some people treat a pantomime shot differently than a dialogue shot. With dialogue, they will relish in the nuances and detail, and try to get every bit of realism into it that they can. Then they will do a pantomime shot and feel that it has to be a super cartoony Charlie Chaplin piece. Don't get me wrong-- I love Charlie, but pantomime should still feel real to an audience. Sometimes going big is absolutely necessary, but there is a difference between that and overacting. You don't want your character to look like they are "playing" sad. You want them to actually be sad. Just because there is no dialogue doesn't mean that every idea has to be expressed with extreme antics. Viewers will respond to something that feels real and honest. The ultimate goal is to have them forget that the character was ever animated. Watch any great live action drama and pay attention to the shots that have no words. You will see that some of the most powerful performances from actors often happen without dialogue. If a person can do it in front of a camera, then I don't see any reason why an animator shouldn't strive to achieve the same thing.
Speaking of cameras, I can't emphasize the use of video reference enough. I always like to start my planning process in front of a camera, because you can get some surprising results from just getting up and doing it. It is the quickest way to explore ideas, and it will very often give you the idea that otherwise wouldn't have occurred to you. Also, video reference is incredibly useful as a tool for good body mechanics. There is no dialogue to distract the audience from what is happening in the body, so they will notice every little movement that is slightly off or awkward. Pantomime is very unforgiving in that way, because the animation is all they have to focus on. The video reference will give you a great foundation to base your animation on.
A huge part of creating believable, lifelike characters is the face, and more specifically, the eyes. Eye lid positions can tell you how alert someone is. How often a character blinks can convey intensity, or insecurity. Pupil dilation can show excitement or fear. Eye direction can reveal intent or help anticipate an action. Subtle shifts and saccades can hint at an internal thought process. Blinks can be used to show a change in thought or emotion. The list goes on and on. The eyes can communicate just about everything, and (assuming there is no nudity in the scene) the eyes are where most people will focus their attention. They are also one of the biggest areas of neglect in animation. Make sure that the eyes are moving like a real eye would, and not how you imagine an eye would move. Do some research on eye movements and you will probably be surprised at how complex it really is. I have some websites that I regularly use as a refresher on eye movement because there is a lot to remember. Everyone has heard the complaint that CG characters are "dead in the eyes". At times that may be partially due to how a character is lit or rendered, but I think a lot of it also comes from how they are animated. It is vitally important that the eyes not be an afterthought in your animation.
To sum it all up:
-Strive for clarity in your pantomime so that an audience can clearly understand both what the characters are doing and also what they are thinking and feeling.
-Focus on strong, clear poses that are both aesthetically pleasing and tell the story.
-Make sure that the timing in your shot helps the character feel like it is thinking as well as doing.
-Give the eyes plenty of attention in your animation, because they can totally sell your character's internal process.
-Make sure that your acting choices feel real and not contrived. Avoid the cliches when possible. Pantomime can be cartoony or realistic, but the character needs to be believable in either scenario.
-Use video reference as a tool for planning your shot and as a reference for body mechanics.
-Be sure to enjoy yourself. Shots always turn out better when you have fun working on them