Here is the gameplay videos for the different heroes.
Tracer (Offense): https://www.youtube.com/watch?v=Ara1dYUMGgw
Reaper (Offense): https://www.youtube.com/watch?v=CIupqSnOeOY
Winston (Tank): https://www.youtube.com/watch?v=h_LnGExtkFE
Bastion (Defense): https://www.youtube.com/watch?v=ct5y6m75dOU
Symmetra (Support): https://www.youtube.com/watch?v=w5Dga2y3No0
Thorbjörn (Defense): https://www.youtube.com/watch?v=MFh5HFL1MXc
Widowmaker (Defense): https://www.youtube.com/watch?v=QB5Zxl9j8Ps
Hanzo (Offense): https://www.youtube.com/watch?v=tyPYV2Xu3iA
Reinhardt (Tank): https://www.youtube.com/watch?v=tkFdb1KCTL0
Zenyatta (Support): https://www.youtube.com/watch?v=75tk1awcUcw
Mercy (Support): https://www.youtube.com/watch?v=SVGkHAK011U
Pharah (Offense): https://www.youtube.com/watch?v=cQnpGwzC89c