Hmm, let me expand my point. When I say "Long Term Impact", I wasn't referring to the impact the game had on the gaming world in general. In retrospect, "Long Term
Presence" would be better wording. Basically, I believe Overwatch will more likely have a MOBA-esque community instead of a community like the one TF2 had, and I believe it will suffer in the long term because of it...
(Note: These thoughts are based on watching a lot of gameplay of Overwatch during its last beta phase, so things may have changed...)
There's a couple of reasons I believe this. First, how matchmaking and teams work. There are no community servers in Overwatch, as to be expected in the modern era of gaming, but also when you make a group to play with friends, you and your friends will always be on the same team. This creates the issue that the enemy team is not your friend, and will never be your friend, and if the enemy team somehow does become your friend for whatever reason, the friendship is fleeting. You will never come across each other again within a time frame where you are able to remember who they were. This means that games will revolve around one thing: the game itself. There's also no sense of permanence or familiarity that is usually created by a community server. If you constantly go to a community server, you become a regular and you become known by the other regulars, and eventually you become friends with them. In matchmaking systems, that doesn't happen. Heck, it doesn't happen as much as it use to in TF2 because of the presence of Official Valve Servers. Players sacrifice community for reliability in population. It's very rare you'll make a long-term friend in a Valve server unless you are trying to the moment you join (and even then it's a long shot)...
Second, emphasis on winning. In TF2, what happens when you win? You get ten seconds to destroy what remains of the enemy team, while the MVPs of your team show on screen, and after that the game begins again, sometimes with a minute of preparation time depending on the map, with team swaps and scrambles when needed. This happens about five times per map, sometimes less sometimes more, before the server changes map. In Overwatch, when you win, you get a shot of your team all together, get to see the play of the game (together these take about 25 seconds overall), then you get forty seconds of the post-match voting thing and progression stuff. Then another forty seconds to pick your hero. Then a minute of preparation time. That's almost three minutes between matches, with a good chunk of it praising the winners and MVPs. Then, for each map, you only play it twice, once on offence once on defence, and then you swap map, and your matchmaking record and hidden ELO and whatnot is updated and whatnot...
The heavier emphasis on winning means that players are more encouraged to play to win, like a MOBA, as opposed to playing to have fun. You won't see people doing silly things unless they are in a group with other people who want to do silly things (y'know, from the kind of people who make the funny videos and whatnot), but if you want to have the games measurement of your skill be accurate, you have to play to win. Solo Queue will be a no-fun zone full stop, least you anger the players who do want to win. And since teams in Overwatch are a lot smaller than teams in TF2, each person has a larger contribution, so if you aren't contributing much because you're doing something silly in Overwatch, your team is gonna suffer. It's the same in any MOBA. Winning is important. In TF2, it isn't. I mean, its nice, but it doesn't really mean much, so if you want to just goof off, you can...
This video from Jerma talks about the importance of the upcoming TF2 competitive matchmaking (just a heads up the video has some naughty language). In it, he says that there are basically, like, four or five different kinds of players on a TF2 server, and with the addition of matchmaking, those who want to play seriously can play competitive matchmaking while those who are new or want to goof off or relax just go to regular servers, and players can jump between the two whenever they want. You can't do that in a MOBA (yes, there's a ranked and unranked option, but you are still expected to play to win in unranked and the enemy team will most likely too), and it's looking to be that you can't in Overwatch. Everyone has to play together in a way where winning is the most important thing...
These two points I feel will create a weak community. Just look at any MOBA community. Most of the time it's gonna be centralised in a couple of places. You'll have the official forums, you'll have a subreddit, maybe the odd something or other, but that's it. TF2 has tons of small communities, and had a lot more when community servers were the main form of playing. You have your general ones, you have the ones based for each community server, you have the ones based on each type of modding you can do, so on and so forth, and these each had a great sense of community for itself especially compared to the larger communities seen in MOBA communities, where you'd only be well known if you contribute a lot. You will not be known just from the playing the game or making the occasional comment...
So yeah, that's why I think Overwatch might suffer in the long term. It's gonna be great playing around with all the heroes and whatnot, but I doubt there will be as vibrant a sense of community than there was in TF2...
(Addendum: Also, playing with friends new to a game with matchmaking sucks. In TF2 you just hop onto whatever server and have fun. In matchmaking either you are against opponents above their skill level or far below yours, or a mix of both...)