Why the symbiote event was such a good event
Well, in my opinion at least.
You get a useful champ, but you have to upgrade him. This adds a goal to finishing Master and Epic that gave those quests more of a purpose.
A decent difficulty spike in the content. Golden symbiote sucked a bit the first round of epic, but once you maxed your sym and used the true strike boost, he was a one shot.
COUNTERS! Very often when we as a community get angry it's because of, as Seatin says, fun and interactive gameplay. *cough* Ice Phoenix *cough*, but here we had counters to all the nodes, which made for a fun and strategic event.
There's a reason why people hate getting Iron Patriot and seek out the Sparky's of the game. Besides the fact that Sparky actually hits decently, he has power control and taunt which makes him a great choice for, for instance, all or nothing nodes.
Most of the old champs are in dire need of buffing and this 2 champs every 6 months is not enough. I hope the symbiote event is a start that will change this game and make it more fun again.
(Though the global node AW change seems to be a counter evidence of that hope).