In the past days I've been wondering about math books of the antique, e.g. the well-known Euclid's Elements. I looked at a few pages of the original Euclid's Elements, but of course was not able to understand anything due to the Greek language. However, some basic questions came in my mind:
How would Euclid's Elements (or antique math books in general) compare to modern high school / university books?
To specify what I mean by compare, let me give you some criteria:
The information density. I have seen that the pages are 'filled with letters', but I can hardly image that Euclid's Elements includes more information than a common high school book. Is that assumption wrong? Have the Greek mathematician used many examples? Or did they 'blather' a lot?
The size. If we would reduce the writing to a modern 10pt font size and use the modern page formats, how many pages would Euclid's Elements have? Would it be more like a pocketbook or like a huge 1000 page university coursebook?
Up-to-dateness. Would a good high school student know most of the information of Euclid's Elements or at least a average university student? Would we call the information given in Euclid's Elements basic knowledge today? And/Or could a university math student derive most of the results by himself (since for university students it is very easy to derive e.g. high school theorems)?
When reading papers, even ones that were written only recently, one of the first tasks is to translate what the author did into his own mathematical language, into his own terms. For most concepts, there are many different ways to see it, and different authors might be doing the exact same thing without realizing it at first, as they have different approaches and languages.
If you go farther back and read a paper from, say, 100 years ago, you will notice that it gets even harder to translate it. The results in there might (or might not) be easy to proof given todays knowledge and understanding, but to first see what the author is doing, how that translates into your own understanding and your own pool of knowledge, get increasingly difficult.
Going back even further, one might notice the problem that mathematics changes over time, the terms, the very thinking evolves. To give one example, the topic "group theory" was originally the study of the symmetric group and its subgroups. The axiomatic approach to group theory we know today got introduced much later and one important result (by Cayley?, I'm not entirely sure right now...), showing that every group in the new sense is indeed a subgroup of the symmetric group, allowed this new concept to survive and gain popularity among group theorists of the time, that at first did not care that much about this strange concept.
Another example, way farther back, is the proof by example (regarding your second point). What is considered a rookie mistake nowadays was common in the old days. A theorem got proven by computing it and showing that it is true for "enough" numbers. The whole idea of proving theoretically for all (e.g.) natural numbers got introduced only later.
And last but not least, you should consider that mathematics was part of philosophy for a long period of time. It has still some relations today, even though it got closer to natural science over time.
All in all, the farther you go back, the more different mathematics get. Thus it might be able for a high school student or a university student to derive the results by himself, if he would be able to understand them at first that is. Translating the old works to your own language (and I mean that mathematically, not Greek to English) is in my eyes the most difficult part here - from there, using todays results and methods, many things might seem trivial (and others might not).
Standard disclaimer: This answer is based on what I myself heard and learned about the history of mathematics during my studies, I am in no means an expert on the field.