Since there is no individual gold/item choices the only things that impact the outcome of the match are positioning,team fights(coordination) and map objectives. All of them favor groups so much more over solo play. This is ignoring the hero aspect completely(ie a team of 5 choosing 5 heroes that match up together really well compared to random player's heroes of the week).
There is no doubt the game will be successful. Its a free game+ it has Blizzard's brand. Just not wildly successful.