I recommend playing every class at least once. They all bring something different to the table.
And all classes are so overpowered in this game. It is not just Infiltrator that can steamroll through the game regardless of difficulty.
Vanguard:
But Vanguard is probably the most powerful class in this game, even more so than the Infiltrator. A Vanguard using Charge + Nova (Half Blast) + Incendary Ammo with Explosive Burst is an unholy terror. And even then players have mentioned all you need is Charge + Nova and you do not even need to fire your gun.
You can play it other ways, too, either by ignoring Charge or Nova, or mixing in other powers for a more rounded or caster-oriented approach.
Infiltrator:
Using sniper rifles on an Infiltrator? People still do that? Use shotguns instead. Completely different gameplay style and much more fun in my opinion. I mean, sniping is okay but shotguns are more effective on the Infiltrator than sniper rifles. (Also, the Talon pistol which is a shotgun in disguise.)
And as for you comment "Tech powers are pretty useless too, especially hacking, and only serve as a short time distraction at best." then you are missing out. Sabotage is the best tech power in the game, and the second-best power on the Infiltrator (next to Tactical Cheat of course). With Tech Vulnerability at rank 6, it is the signature tech primer of the Engineer and Infiltrator. Using it for hacking Geth/Turrets is just a bonus.
For Sabotage on non-synthetics, it will backfire about 1.5 seconds after it hits; you can see the sparks when it backfires. The effect lasts for about 2 seconds after the backfire. During this window, hitting the enemy with any direct-damage power will result in a tech burst. This can outright kill groups of enemies or leave them with little health.
The Tech Vulnerability evolution of Sabotage also increases the damage of all tech powers for 10 seconds, including the Backfire damage of Sabotage itself, and will also increase the damage of Tech Bursts. It may also increase the damage of Fire Explosions and Cryo Explosions, but this has never been extensively tested as enemies are usually dead by this point.
Infiltrator with Marksman is also crazy good, and you can bypass the Marksman cooldown by exploiting the Cloak cooldown mechanics.
And Tactical Cloak with the Bonus Power evolution? That allows you to use up to two powers under cloak + shooting with +100% (or more) damage.
Sentinel:
Tech Armor provides constant damage reduction. Add in Fortification as a bonus power to max out your damage protection and you can soak up a lot of bullets. The real tank of this game.
Casting-wise, take Liara (Singularity) and Javik (Pull) and you can steam-roll through mooks.
Unprotected enemies? Liara/Javik set up, you Throw.
Shielded enemies or Phantoms? Overload > Liara/Javik > Throw.
Armored enemies cannot dodge, so just Warp > Throw. You can even shoot a bit while you are still on cooldown from Warp.
There are other ways to play the Sentinel, but those are probably the most effective ways to play it,
Engineer:
Combat Drone and Sentry Turret are okay for distracting, but are rather underwhelming in my opinion and can mostly be ignored.
The bread and butter of the Engineer is Sabotage, Incinerate, and Overload.
Groups of enemies, shielded or not: Sabotage > Overload (tech burst).
Armored enemies: Sabotage > Incinerate (Tech burst) > Overload (fire explosion).
You can also continue alternating Overload and Incinerate (or Incinerate and Overload) against really large enemies to alternate tech bursts and fire explosions.
There are other ways to play the Engineer but that is the probably the most effective way to play it.
Soldier:
Adrenaline Rush + Incendiary Ammo with Explosive Burst.
Wreck shop.
Concussive Shot is an amazing detonator (detonates tech bursts, fire explosions, and cryo explosions).
Adrenaline Rush will automatically reload most weapons when activated; for the Prothean Particle Rifle and Lancer it will automatically begin replenishing their ammo instead.
Soldier is also a great sniper (shoot > Adrenaline Rush to auto-reload > shoot again). It can probably kill faster with a sniper rifle than the Infiltrator can.
Adept:
Play pretty much like the Sentinel except you can set up your own biotic explosions with Pull or Singularity, which allows you to snag more than one enemy at once to be detonated.
Personally I think the Adept is the weakest class, but it is still super strong.