The Frenzy Virus is my personal favorite "powered up monster" sort of mechanic and there was much more to it than raising the stats of the monsters.

Frenzied monsters attacked faster, dealt more damage, didn't get exhausted, had their roars enhanced, and made the AI far more aggressive and brutal to fight. You can also knock monsters out of the frenzied state which staggers them and often causes a shiny to drop on the ground.

On top of that, the player could get infected which did a lot of bad things to the player such as nullifying health regeneration, but if the player overcame the virus and kept attacking, they temporarily become immune to the virus and get huge stat buffs.

Frenzy is a great mechanic because it teaches you to be aggressive in combat. The best way to fight the monsters' aggression is to be aggressive yourself. This isn't just because you get a buff for overcoming frenzy, but you'll get totally run over if you let a monster push you around.

Apex, the second stage of the frenzy virus, is when the most powerful monsters such as Rathalos, Deviljho, and Zinogure overcame the virus and are empowered. I'll admit, Apex monsters were more infuriating to fight than fun since all of your attacks would bounce off of the monster if you didn't buff yourself with an item that lasts for a short amount of time, hitting the weak spots of some monsters like Deviljho and Rajang was a total pain.

As bad of memories as I had with the Apex state, I think that it's far more interesting in concept than tempered monsters and it really rounded out the "lore" of the Magala family much better than whatever Tempered monsters is meant to be.