Never done a critique before, but ill do my best
For starters I love how you did her stance. The way she leans shows how she's trying to heighten her status, making her seem powerful and something to be feared. I also love her mane and the way it covers some of her face. This shows me that although Fluttershy means business, she is quite shy and has low self esteem. This leads to me connecting with the vector by saying, "IM TOUGH STUFF! Sorry if I yelled..." That brings out Fluttershy's innocence -and cuteness- giving of a big impact to the person looking at the vector.
Now for the one bad thing; Originality. A lot of people do angry Fluttershy vectors for lolz and showing how Fluttershy isn't weak and helpless. The one scene in 'The Best Night Ever' made a lot of Bronies see the potential in Fluttershy for fanfics and angry vectors. If you did this free hand then congrats on the facial expression. If not try be adventurous and do a little free hand. Hope this was good and helped