The menu is mostly Thai classics that have been updated and refurbished a bit, so the result isn't so much fusion or nouveau Thai cuisine as just really, really good Thai food. The restaurant has a modern New York feeling to it -- obviously someone spent a lot of time and money making it look good, but the result is a very casual and comfortable space.
Perhaps the coolest part about Soul Food Mahanakorn is that it was opened by an American food critic, Jarrett Wrisley, who apparently got sick of critiquing other peoples' restaurants and decided to open his own. He has a great blog that chronicles the trials and tribulations of opening a restaurant in Bangkok (plus there are lots of great food photos!).