Now after having Rhythm and Pattern/Spacing Cover, let's talk about Low difficulties.
By Low difficulties, I am mainly talking about Easy, Normal, Hard, and Sometimes Insanes (Depending on the insane.)
Low difficulties, unlike top difficulties, usually have more stricter standards when it comes to ranking, mainly for the obvious fact they are for lower difficulty players.
Whilst RC comes more to play and considering the stricter standards around it, mistakes are highly prune to happen!
What to look out for when modding lower difficulties.
For Normal's and Easy's difficulties, there is a lot of things to considered:
The amount slider usage vs circle usage
Rhythm Density within sections
Readability issues which go hand to hand with AR and Spacing.
Slider Velocity (SV) changes
Stacking usage
Spinner usage
Slider shapes
Overlaps
Whilst there is other things to look out for when looking upon these lower difficulties, these are the main things you should be looking for at all times depending on the mapper's skill level, song, bpm, and other things that I will mention in the section [Spread]
For Hard, similar things above apply but mostly more lenient, similarly for insanes but even more lenient.
Another thing to keep in mind is the bpm of the song at hand (is it 180, is it 160?) since usually that makes things much lenient and some things that may be a problem on higher bpm, could be less of a problem on lower bpm.
Here is the link for it: https://osu.ppy.sh/help/wiki/Ranking_Criteria/Scaling_BPM
Upon finding issues in low difficulties
For the most part a lot of things you see around lower difficulties can be easy to find especially upon the guidance of the ranking criteria (https://osu.ppy.sh/help/wiki/Ranking_Criteria/osu)
But of course just relying on the Ranking criteria alone isn't going to help and even just having a list of what things to look out for, this can be quite daunting at first.
In general which I will then repeat myself like always, Go through the entire map! You do not want to be posting issues within a set and not even know how the rest of the mapset looks like.
(Like repeating yourself over and over to the point it all could have been summarized i.g.)
It will always be a good starting point. Then afterwards considering what I have said and things that the ranking criteria said, try to think about some things when looking at some patterns in a mapset:
Is it causing a reading issue?
Usually for lower difficulties, reading issues are a fairly common problem that could be just simply the difficulty settings. By that I mean Approach Rate (AR). being lower than it should for the map.
For AR this issues differiates from bpm and density of the map. For maps higher than 180bpm and is fairly dense, then using AR 5 - 6 is more preferable, and lower than 140 and the song isn't particularly that dense, AR 4 - 5 would be easier on the eyes Just to give some examples.
Others could be the stacking of the map. Usually this is more problematic in Normals as generally Easy use mostly 1/1 rhythms whilst hard this is pretty lenient and is simply dependent on context.
In Normals you have usually 1/1 and 1/2 rhythms here and there and there is that occasional time a mapper would may or may not unintentionally map 1/1 stack and a 1/2 stack in similar time periods.
For sliders it isn't bad on the player as there is a little more leniency, but it is bad on circles such as this:
(on the left 1/2 on the right 1/1)
As you can very well see the issue here, it is extremely difficult for the average player to recognize and should be pointed out.
Whilst that is more on the extreme side and likely not to occur as much as stacking 1/2 and 1/1 on sliders, in general this is just simply something to consider when thinking about lower difficulties.
Lastly Slider Velocity (SV), usually it is best to avoid SV changes to a degree that is problematic even for Hard level difficulties.
Like for example, you don't want to be seeing a normal that is mostly 1x SV throughout the entirety of the map to suddenly have this one slider going 1.3x, regardless if it is to emphasize the song or not. A healthy reasonable amount of SV changes if the mapper must have it is by sections (1x in one section then 1.1x if song intensity goes to the next i.g) is perfectly fine as long as there is a reason and it is consistent.
Others suggestions on readability sliders that have circles too close to each other, distance errors, is also something to consider.
How is the Slider usage vs Circle usage?
This comes to play in mostly Easy and Normal difficulties on Slider Usage whilst Circle usage could more or less be more iffy around Hard difficulties.
Sliders are usually harder to play than Circles for an Easy/Normal player as they are just getting into the game and more likely still unsure how to fully play it out. So with that being said, you don't want long periods of sections having a lot of sliders in it.
Try to keep in mind about bpm however, even when sliders are going on around 10 seconds, at low bpm like let's say 130 - 140, it could be seen more minor as there is more breaks in between where as 180 bpm, this could be more problematic especially if it is a regular occurrence within the mapset.
Circles being used longer than 10 seconds can also be problematic at times but usually, that is more rare and less likely to happen as most people have a more mentality that circles is harder which is likely due to their skill range being out of the low difficulty range.
Does it have Rhythm Density issues? If not contrast?
Rhythm Density issues can be harder to notice on first glace on higher difficulties, but they can be easier to notice on lower difficulties especially those even experienced that doesn't particular map a lot of low difficulties.
There is times where sections could be way too easy while sections can be way too hard for say example you go into the kiai on a Normal difficulty and you notice a lot of 1/2 slider usage with not many breaks at say 160 bpm, or the kiai is reasonably dense but then you go back and notice the other sections is mapped similarly or a pattern being much harder than the overall intense kiai.
There is many factors to consider around Rhythm Density and Contrast but simply take one step at a time (and if you look through the map over all, it wouldn't feel so overwhelming once you get used to it)
There are other elements to consider such as ds errors, screen usage, etc but they are more or less get on the misc. side fairly easy and nothing too much to worry about to point out unless easy to notice!