In my opinion, I think it's fine without a hero limit - it's the way the game is designed. It's not a MOBA and it really gives a dynamic feel to the game in situations in which it works. Not to mention having more of the same hero on one team is more a disadvantage most of the time, than advantage. The freedom to have more of the same heroes on one team I think will really make Overwatch interesting, especially at an esports level. Every hero has a counter, so being dynamic in the game is one of it's best elements, sometimes the push might be to have a few of the same heroes and if it works, great, adds to the element of suprise.
Lots of MOBA players, who played League / DOTA 2 have the opinion that you shouldn't have more than one, or not changing at all (which is what the game is solely designed around)
Just my thoughts

of course not everyone will agree, and some will