A sentence should express an idea. It may be a simple idea or a complex idea, but it should be only one idea. It should be unified and focused. That's the whole reason we separate our writing into these units. Often, authors try to cram multiple ideas into one sentence in an attempt to quickly get a bunch of information across. But if you stick random elements into a sentence, you end up with a very weak sentence. For example,
The kingdom, which I built and named Gru, still knows the peace I bought with my own blood. The entire island continent, large enough that on horseback you could ride more than a week to cross it east to west, is united under my banner still.
In the first sentence, the information "and named Gru" has nothing to do with the main idea of the sentence. The sentence is conveying that he built the kingdom and made it peaceful, and those benefits are still being felt. The author is trying to jam in the name of the kingdom because he knows he needs to give it somewhere, but it doesn't belong here. Including it knocks the whole sentence off balance and we lose the focus and the point. The second sentence is phrased as if it is two separate ideas, which makes it very difficult to follow and prevents the reader from getting the meaning the author wants. In this case, the sentence is actually trying to convey a single idea, but the author hasn't phrased it that way. He's phrased it as two separate ideas: (1) the continent is large, and (2) the continent is united. The real point is that despite the large size of the continent, it remains united because of the narrator's efforts. Yet the author hasn't connected the two parts of the idea to reveal that it is a single idea. The author needs a structure that does this, something like,
You would have to ride more than a week to cross it east to west, yet the entire island-continent remains united under my banner still.
Let's look at another example:
Joe did not want to deal with the issue of pollution caused by the factory that now occupied the former warehouse on the Merrimack River.
Again, the author is trying to put two ideas into one sentence. One is that Joe doesn't want to deal with the pollution. The other is that the factory is in a former warehouse. Putting these in one sentence creates an unfocused, awkward sentence. These need to be in two different sentences. The current sentence also doesn't believably reflect what Joe would be thinking. The author is jamming in extra information that he wants us to have. Joe would probably think about the location of the factory in some later paragraph.
Here's another example:
A slender, attractive Chinese woman about forty years of age with short, black hair, in an expensive business suit walked out, head held high, with a purposeful stride and the confident manner of someone used to being in authority.
You can see how awkward these sentences are. The author is jamming two ideas into one sentence: describing the woman's appearance and describing her actions. This should be two separate sentences. (The sentence is also awkwardly phrased, so that it sounds like her hair is in a business suit.)
Instead, we might write,
Inside stood a slender Chinese woman about forty years of age, wearing a perfectly fitting black business suit. She strode out with head held high, short, black hair swinging against her suit collar, hands clasped with the calm, confident manner of someone used to being in authority.
This way, the description is split between one sentence of her standing still, and one sentence of her moving. That helps create focus.
Okay, one more:
A pouchy woman with a crown of frosted coppery ringlets, Sela wondered about her friend.
Can you tell what the two ideas are here? The author is telling us (1) what Sela looks like and (2) what Sela is thinking. The author seems to be combining things randomly--probably in an attempt to create variety in her sentence structure. She has a further problem in that the description of Sela is told from an external point of view, not from Sela's POV. She wouldn't think of herself as "A pouchy woman with a crown of frosted coppery ringlets." Yet the thought is clearly internal, coming from within Sela's head. To separate the two ideas, let's first make two sentences:
Sela was a pouchy woman with a crown of frosted coppery ringlets. She wondered about her friend.
Each sentence conveys only one idea, but the POV problem still exists. To solve that, I would change the description of Sela to make it from her POV:
Sela pulled anxiously at her coppery ringlets. She wondered about her friend.
At this point, we could even combine the two ideas to make them one complex idea, meaning one sentence:
Sela pulled anxiously at her coppery ringlets, worrying about her friend.
Now the entire thing is focused on showing Sela's concern about her friend, so it is one idea and can be one sentence.