here's a great read to focus on improving your animation skills-
http://art-eater.com/2010/07/test-1-darkstalkers/in the article are "12 key principles" of animation that are applied to Darkstalkers. Out of the 12,
2, 5, 6 and 9 seem most relevant to what you could improve on.
"2.)
Anticipation: this is the build up before the release of the action. Since this telegraphs movement, you can’t have too much of this in a video game as inputs should be fairly instantaneous."
in your sprite, there's no anticipation that hes about to swing the sword. he just does it. like it says, a game attack needs to be fairly instantaneous, but anticipation could literally be milliseconds and still be incredibly powerful, especially when you take liberty to exaggerate.
"5.)
Followthrough and overlapping action: This mostly pertains to things that trail the main action like clothing, hair, bouncing breasts etc, think of it as the aftermath of the main action. The Vampire games are good about this. DarkStalkers is one of the first games I can recall that has any followthrough animation at all."
I feel the scabbard's weight is not portrayed well and could help sell the weight of the swing if you consider how the scabbard would react. i think it would flap up slightly.
"6.)
Slow In Slow Out: This refers to changing the spacing and timing of actions so they’re not totally uniform and flat...The frame padding in Vampire is very expertly done to give weight to the moves. Fierce attacks tend to have alot of frame padding, giving them a very meaty feeling. They looked and FELT stronger than weak attacks.
Oh and I almost forgot to mention, another thing the Darkstalkers games do well is
the characters tend to lead their actions with their shoulders and hips...This is supremely important for imparting impact. It’s a basic principle of martial arts that your power comes from your core. Alot of animation still does not utilize this principle. Many 3d games have animations where the characters lead with their extremeties rather than their body, leading to a feeling of a marionette on strings. 
i think number 6 is one of the big ones. you need to consider timing of the swing. i just did your swing in my room (albiet i generated more power by leading the action with my hips and core strength as is described). when i did the swing, i noticed a very brief startup, then a huge explosion of acceleration generated by the pelvus/core and this speed followed most of arc the swing. then lastly was a quick cooldown period directly after the explosion that followed into the very very end of the arc to maintain a smooth and balanced motion.
9.)
Timing: This refers to the speed of an animation. Timing helps to establish physical properties such as weight, velocity and force. Timing is also important to acting. A character who is depressed might move very slowly with a lot of inertia. A character that’s scared might make fast, jumpy motions. Timing can be very abstract. A humorous animation could be said to have good comedic timing.
this reinforces number 6. rethink timing to go with the intended power.
lastly, i think youre being too safe. you may discover that you could exaggerate and hyperbolize like mad and actually develop an animation that looks MORE natural than less.
the stretching looks ridiculous, but when you see the actual attack, the timing, followthrough and other elements make it look a lot more natural than you'd expect!
