So I was thinking about the games balance and team composition, and this has also lead me to consider why some of the cast are pretty lackluster. It seems that pretty typical team composition right now for competitive (tournaments and such) features two tanks, two supports and two flexible roles.
So successful teams will often be something like this
Winston
ReinHeart
Mercy
Lucio
McCree
McCree
The flexible roles are more interchangeable but the support roles are relatively constant. Lucio's speed buffs do not stack therefore two Lucio's are not good, Zenyatta isn't very flexible so he gets focused down too easily, and Symettra isn't doesn't contribute to team sustain in the same capacity that other characters do.
Tanks generally switch between Reinheart and Winston, with stacked Winston compositions being popular despite the nerf to his shield since the beta, it's still a powerful combination and very capable of focusing down squishy characters while controlling the upper areas of the map.
Concerns regarding defenders
Unfortunately, this style of team composition isn't very interesting in my opinion, while one would think that the attacking and defending characters would make up the core of the team on the attacking and defensive modes, that does not appear to be the case at the moment. Defenders especially are very lackluster and underutilised in competitive play. Taking a look at the competitive usage from the beta, four of the defensive characters were within the bottom seven least used characters.
I think the problem with these characters and the reason that they are not used often at higher levels of play is that their 'defensive' role is something of a lie. Characters like Hanzo and Mei are not remarkable at area denial, and the vastly more flexible attacking characters just ignore their defensive efforts. Genji or Tracer and repear have no difficulty isolating these squishy defensive characters and the end result is that they are often a liability. Even Widowmaker, the only defensive character with a reasonable level of usage is very difficult to make good use of without both an incredibly coordinated team, and a very high skill level from the individual player.
I think a big part of the problem here is the fact that while these characters are labelled as 'defenders' they are not the ones that make the bulk of the defensive meat. Tanks exist as the defensive wall and these are the characters that end up pushing the offensive teams back, and holding the defensive line with both damage mitigation and bot the damage and utility to make an offensive push.
Combned, these tanks and attackers occupy most of that which would be required from the defensive team. They can form an incredible obstruction, and flexibly output very high amounts of damage without feeling as though they present as significant a liability when focused down. A McCree is a better pick than a Hanzo because when Winston does jump at your team, McCree can stun and instantly-kill him, Hanzo and Widow fold under any degree of pressure and they're dominantly long-range orientated weapons make them very hard to utilize on some of the objectives.
If some of the more defense abilities like shielding were placed on these defensive characters they would have been more significant in crafting the defensive wall that ultimately facilitates control of the objective area, but as it stands, they're just squishy targets that offer rudimentary area denial that is often better covered by the more flexible attackers. McCree's stun makes him functionally a better defender than Hanzo will ever be because of his ability to put a halt to and immediately focus down aggressors.
Widowmaker largely escapes these concerns by being a rather generic OHKing sniper, immediately placing her in a class beyond her peers merely because the character archetype she mimics necessitates extrodinary accuracy contingent damage output to be a component of the characters design. If I were to suggest a resolution it would probably be to provide, Junkrat and Mei with a little more health (50) so that they could provide a more substantial component of the team, enhancing their defensive presence, and I would enhance Hanzo's projectile speed (and fix the forgiving hitbox) so that he possess a higher skill ceiling.
But, I don't know, I feel that something needs to change with these characters and perhaps orientate the team less towards double support & tank anchor so as to facilitate more flexible team compositions too. In either case, the cookie cutter 2/2/2 team comp that's present at the moment renders characters in the support and tank slots over represented. The other possibility is that the meta will just change on its own, but I have doubts as to whether it's liable to shift at all towards a higher utilization of defenders, especially when success in so many of the gametypes feels contingent on mobility and momentum.