These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Catharsis Factor: There's an odd satisfaction in destroying large swathes of enemies en masse.
Goddamned Bats: Mutation Agents. Looking like a weird cross between a rock, a tumor and a world war two sea mine, they show up in mid-stage horde areas and puff out a marginally damaging black gas around themselves regularly. Then they start upgrading nearby enemies into Elites. These would easily rank as Demonic Spiders if it weren't for the fact that they upgrade enemies at a fairly slow rate.
Any enemy that locks down a hero is also a Goddamn Bat due to the afflicted hero requiring an ally to come and help them break free, Left 4 Dead style. If your co-op partners are a bit slow on the uptake this can go sour very very quickly, given regular enemies won't stop beating on you while you're locked down. This also means going off on your own in a co-op mission isn't such a good idea. Thankfully, these enemies don't show up during solo play.
Anything with Persistence, signified by a glowing yellow halo above their heads, will get straight back up if killed. Fortunately, they are generally only able to do this once. Things with a Persistence Aura on the other hand, are out and out Demonic Spiders.
Scrappy Mechanic: All game information is stored server side. Therefore you must have an Internet connection to play Single Player.note A server outage in mid-2013 caused a panic that EA had dropped the game and it would be unplayable; fortunately, it was simply a technical issue. EA usually publically announces any intention to shut down a game's servers. Also, though not as bad, is that it's impossible to create a new save. The closest is to delete all of your items, but that still leaves all unlocked creatures and Crogenitor levels.
They Wasted a Perfectly Good Plot: An action based Spore game that will prove to be better than Galactic Adventures? Sounds awesome! Now, where do I build my monster? Wait, I can't make one from scratch? I can only use the preset ones? FFFFFFFFFFFFFFFUUUUUUUUUUUUUUU...
The thing is, Maxis never claimed that Darkspore would be a "better version" of Spore. Right from its announcement, it was marketed as a completely different game. All the hype you mentioned was inflated by the fans.
More to the point, they never took full advantage of the fact that the editors are based on the same engine. It would have been great cross-promotion to allow people who had both Spore and Darkspore to build new heroes in Spore to import to Darkspore. There's a mod called "Dark Injection" which allows you to use a bunch of parts from Darkspore in the original Spore, so if they'd wanted to they could have found a way to get the games to work together.