Let's say there is a slimy floor aspect in the scene. When exactly would this property of the floor be used?
Why it's important for me? In the first case aspect is a "part of reality", in the second, aspects appear only if someone has fate points. No points, no features.
As narrative truths, aspects can and influence the story being told. So whatever anyone does at the scene has to take the slimy floor into account. If someone says they are running across the room, just ask them how they are dealing with the slime. Maybe they have high traction shoes so it isn't a big deal, or maybe they want to overcome it with their high athletics.
So, first and foremost, what an aspect does is introduce questions and complications where there would be none.
Compels and invocations come into play only when there is a roll involved, and then only when someone wants to make that fact significant for the situation.
So you could see it as a three tiered methodology when applying aspects.
Both are true at the very same time. The door is slimy and all the people have trouble moving. If an action seems unbelievable on slimy ground, it doesn't happen.
Invoking or compelling only happens when the camera zooms in, and if it is really important for the story.
Aspects don't simulate reality. Fate simulates story. So check if the aspect is important for the story - if it is, it's worth Fate points.
It depends on what characters are trying to do. If the slimy floor is the obstacle, then it's there without fate points, but it's the narrative detail and not the aspect that matters. If the aspect describes something about that would make a task easier or more difficult, for example if characters were fighting on the slimy floor and it was really important to stay clean, then the aspect only exists when people pay fate points for it, and even then may only exist convincingly if a margin of success sufficient to give someone a consequence is achieved. This is because of how Fate Core integrates aspects mechanically-- while they can be used descriptively to justify outcomes whenever a player wishes, they only actually influence outcomes when points are spent. Furthermore, spending points results in a numeric modified applied to the roll; aspects don't completely modify how skills work or replace them with different skills-- stunts are for that.
This means that a Slimy Floor aspect is possibly not what you are looking for to model the increased difficulty and extreme lethality involved in the above cleanliness-based melee. While an aspect may be useful as an additional feature, other systemic changes (like changes to how consequences work in this contest, an additional Cleanliness stress track, a pervasive -2 fight modifier, etc) might be much more important to making the floor detail 'real'. What exactly is needed will depend on the situation, but I generally require in my Core games that every situation aspect has an associated stunt everybody in the area has, like: "slippery floor: you take a -1 to rolls where floor traction is important".
