I appreciate what you’re saying. Here’s a few pointers that are specific to your shot…
1. Weight, Timing and Spacing
These are the core principles on which animation is based, and having a thorough grasp of these principles is what differentiates good solid, believable animation from bad, floaty, computer-y animation. This is the kind of thing that you’ll get better at by observing the real world and practice, practice, practice. Animators will spend their whole lives working to improve these core skills.
I’d suggest getting hold of The Animator’s Suvival Kit, by Richard Williams. It’s a great book that covers these principles in exhaustive detail.
It’s difficult for me to quickly sum up how this relates to your animation because there’s a whole host of things going on. Try to think about the weight of the head, and whether the character is in a balanced posture. His head is big and heavy compared to his thin body, so when he tilts it from side to side, it’s not going to take a lot to make the character feel as though he’s off balance. When the character changes between poses, try to think about how he’ll subtly accelerate and decelerate between those poses, and think about the anticipation and overlap in those posture changes. Also think about the anatomy of the character, what body parts will move in synchronization, and at what point are joins reaching their anatomical limits.
Something that really helps with this is to film yourself acting out the motion and be mindful of the core animation principles as you’re watching it back. Also, try taking a sketch book, going to a park, a coffee shop, a library, or some other public place, pick a random person to watch, and draw thumbnail sketches of that person for 10-20 minutes or so. Limit yourself to 10-20 seconds per sketch, don’t worry about the details, just get the broad posture and body language. This type of observation really does help.
2. Body Language and Framing the Shot
Say that all your weight, timing and spacing were all perfect. A big problem that you’re left with is that your character’s body language and the way the shot is framed up close on his shoulders, just isn’t very interesting.
A huge part of how people communicate is through their body language. People use their hands and posture to express what their saying, just as much as their voice. By cutting the arms out of the shot and expressing only with the head and shoulders you’re limiting the amount that this character can emote. I’d pull the camera back and consider how your character is going to express what he’s saying with his hands and body posture. If you’re thinking that body posture probably isn’t that big a deal, check out how expressive Frank Oz is able make the muppets when he only has body posture to work with (http://www.youtube.com/watch?v=BeuekMbXCIw).
Again, filming yourself acting out the dialogue can really help here. Think about the character: What’s his story? What events have lead to this point? What are this guy’s personality traits? Who is he talking to here and what’s the relationship? A good example I once heard with this is to think about a guy walking up to a mail box, posting a routine bill payment and then walking away. Now think about how that guy’s body language changes if it’s not a bill payment but a letter to tell a close friend that he’s been in love with her since they first met. Does he clutch the letter close like it’s a valuable secret that he has to protect? Is he reluctant to post the letter? What if she doesn’t feel the same way and it ruins his friendship? Does he have to psych himself up to post it? When the letter goes in the box, does he feel relief? A sense of nervous excitement? Does he regret posting it? All these questions will have a massive bearing on how the character acts.
Just one pre-emptive word of warning with body language and this may seem obvious when I say it, but I see it time and time again: Try not to be too, “on the nose”. By that I mean don’t spell out every word with an expression, like Hi, I love you. You can be expressive and still show subtlety. 
3. Cadence and Energy
I think one aspect of your shot that’s making it seem a little weird is that it’s not straight dialogue, it’s a music track, and so that makes it feel as though the character should have more energy and life about him. When hear the audio, part of me imagines that the delivery should be a bit more like Victor Navone’s Alien Song. That’s a stylistic choice, of course, but in the same way that the 11 Second Club article suggests that a character’s energy should match the dialogue, it feels as though the background music in your audio clip is part of the energy. In the end it’s a subjective thing so you could go either way with it.
You may be better off starting off with a clip that has more expressive dialogue so that you can really go for it with your posing. Anything from Back To The Future tends to work really well, especially Doc Brown dialogue.
Either way, your character needs to move between stronger key poses and those poses need to match the cadence and rhythm of his speech. You don’t have to hold poses for too long, and at no point should a character stop moving completely (unless him being frozen is part of the story), but you should consider where the beats in the dialogue are and work it so that there are similar beats in the body movement. Currently there’s a sense that the dialogue and expressions are out of sync, or even that different body parts are out of sync, so working a bit on that will help.
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So anyway, I hope that’s been helpful. I was trying to keep feedback specific to this animation, but in general, I would go back to the basics. If you have the bouncing ball down, I’d try doing some of the animation mentor tests that you can see here. They’re really great for practising the basics.
Do also remember to check out The Animator’s Survival Kit, and keep practising and you’ll get there.
