Weapons with literally no downside. I’m not talking weapons like the Wrangler, where the downsides exist but are negligible in comparison to the upsides (indeed, sometimes the only downside is “you don’t have the stock weapon it replaces.”) I’m talking weapons that literally just don’t have downsides and are just better then their stock equivalent.
Not counting ones caused by bugs or bad coding, there have only ever been three straight upgrades added to TF2, and one has since been rebalanced. The Amputator on release granted the healing taunt for free before being changed (for the better in my opinion) on November 21, 2013. The other two, the Solemn Vow (User can see enemy health) and the Third Degree (All players connected via Medigun beams are hit) have been straight upgrades since added to the game.
This is naturally a touchy subject. One of TF2’s cornerstones of balance is that unlocks come with upsides and downsides, even if said downsides are laughable like No random crits or 20% slower firing speed. The point is all to prevent “power creep”, where newly released items slowly turn out better than the stuff that came before. This only really becomes a conundrum with melee weapons, which you’ll notice all three of the straight upgrades are. The problem is that stock melee for the most part just isn’t very good. It wasn’t designed to be; its predecessor the crowbar from Team Fortress Classic was an emergency weapon so bad that you’re better off running for your life than actually trying to get a kill with it. The stock melees in TF2 are certainly better than the TFC crowbar, but they still were made with the “emergency weapon” concept in mind, and a lot of classes don’t need that. The rocket launcher and flamethrower are both far better choices for immediate range and scouts have no reason not to just scattergun a melee-range enemy. Basically the bottle, wrench and knife are the only three stock weapons that actually have remained a useful addition to an optimizing player’s loadout. So why should Valve bother balancing melee weapons to the other stock melees, and instead balance using the “best” melee unlock for each class as a base?
What’s really odd to see is that even in light of this, not many of the straight upgrades are even that popular. Before November 21 2013 but after the removal of the Medieval Medic item set bonus, the Amputator was abandoned by its most fervent supporters (myself included) in favor of the Ubersaw or Vita-Saw; both fundamentally superior options in spite of their minor downsides. The Third Degree is (and always will be unless they change the stats) a piece of junk you shouldn’t use anywhere outside MvM. In 3000 hours of maining Medic I have never once been indirectly killed by a Third Degree because intelligent Pyros will use their Flamethrower or any of their various ways to gain 100% critical hits in close range. The Solemn Vow is actually okay, but its non-combat upside doesn’t make it the flashiest weapon, and certainly few consider it overpowered. The thing is that these weapons are straight upgrades to junk, and who’s mad if things are better than junk?
The real sparks would start flying if Valve ever added straight upgrades for primary weapons, like the Deflector. But I don’t think Valve would ever do something like that…