So, I'm going to analyse Sombra, her kit, and her likely counters.
Translocator:
Very much reminiscent of Unreal Tournament's translocator weapon, right up to being able to teleport to it while it's still in flight. I'd see this being used as an escape tool, as her cloak is immediately canceled by damage. That said, some footage has shown that it could be used to bypass a choke point, like Volskaya last, I think? Compare this to the Spy in TF2's "Dead Ringer", as it is best used as an escape button but can also be used to flank.
SMG:
It appears to be pretty strong, from the footage shown of its use. It's quite short range, not unlike Tracer's blasters, though it has 60 rounds to Tracer's 40, and a lower fire rate. Damage is probably slightly higher to compensate. I don't see high end players relying on the SMG. Her other abilities are very much utility, and I'm pretty sure the SMG is just in so Sombra's not totally unable to attack.
Hack:
The uses of this are twofold. First, you can disable player abilities for a few seconds. This can be used to block ultimates, as was shown in the "introducing" trailer; or it can be used to assist teammates in bypassing chokes like Reinhardt and Zarya, both of which rely most significantly on their abilities to tank damage and protect team mates.
This is likely to cause Roadhog and maybe D.Va a spike in popularity, as they can tank without being reliant on shielding (D.Va because she's going to get more health in the next update, and she can *maybe* booster away before a hack completes, and Roadhog because of his sheer bulk and damage, even if he's locked out of hook or heal.)
Sombra's presence is likely to be able to remove Zarya as a must-pick, because Zarya is entirely reliant on her two abilities to put out half decent damage and protect teammates.
Additionally, I could see this hurting Zenyatta, Lucio and Mercy's viabilities: mercy's healing stream is an ability, as is Lucio's Amp It Up and Boop, and Orb of Harmony / Discord are also abilities. This would probably improve Ana usage as she is the only healer whose primary fire heals.
The other use for Hack is the ability to disable Torbjorn's turrets, as well as snatch health packs; Hacking health packs marks them as team-only and increases their spawn rate. This could be strong, though i don't think disabling health packs is going to be a particularly useful thing where healers are concerned. It's most likely to affect characters who need to flank, besides Reaper (who can heal off his kills) and Tracer (who can recall her health if she's not hacked).
Thermoptic Camo:
Speed increase, invisibility for a short time. Disrupted by self abilities, attacking, or taking damage. This is absolutely not an escape button as it is far too easily disrupted. It appears to be more useful as a method to assist flanking.
Going along with the "disrupted by damage", we can infer who will be able to "spycheck" most effectively. Mei is probably going to be her primary counter: let's say a wounded sombra cloaks and runs away; a mei could wall her in and spray with the cryo blaster, with its large hitbox and high tickrate it's likely a Sombra in the area would be revealed very fast. A frozen Sombra is a dead Sombra, and Mei's 250 HP is enough to survive a close range burst from the machine pistol. Short of having a translocation up, Sombra is useless against Mei if she's revealed.
Additionally, Winston and Symmetra, on simple account that their attacks lock on in a cone would be able to easily sweep an area. Winston's bulk likely lets him survive an encounter, while Symmetra's going to want to hang around her turrets; she just doesn't have enough bulk to do much.
This may also improve Symmetra usage if it turns out Symmetra's turrets target and reveal a cloaked Sombra. There has been no evidence to suggest this is the case, but given that Symmetra is by far the least picked hero, she needs some kind of hero to counter.
EMP (Ultimate):
This sounds, quite frankly, like an absolutely insane ult. Cloak in, get behind a defensive line, hit Q. Sorry Lucio, your ult is useless. Zarya you're now a squishy just like the rest of the cast, Zenyatta is practically dead the moment it hits.
EMP is an AOE "remove all shield and barrier in the radius and also hack opponents". While Reinhardt may not be as badly hurt by this, as he's a big healthy bloke with sizable armor and no shielding, he does lose access to his shield.
EMP will probably also block ults, or shut down channeled ones, which prevents the easy "lol transcendence" defensive ult so often seen in high level play.