When I was a kid the education department printed a school magazine for us schoolkids and it was printed by letterpress and exhibited those characteristics. We kids used to often run our fingers down the pages to feel the impression of the type (sometimes we'd even shut our eyes and try to guess words from the feel of the impressions).

I would dispute your indentations comment for these reasons:

• Bad letterpress is quite ugly and uneven. This has always the case, problems are caused by many factors but mainly from cheap and overly absorbent paper stock which often leads to a considerable variation in print density and sometimes to excessive ink bleed.

• I knew two printers each from a different printing establishment who specialized in letterpress and they lectured me long and hard about controlling pressure to achieve the correct impression depth which is quite critical for excellent work. No impression or an overly shallow one (known as barely 'kissing' the paper) is unsatisfactory, likewise so are deep impressions caused by 'biting' the paper too hard (this also wears and damages the type). In practice, the optimal impression depth is often just past the 'kissing' point. (Incidentally, the reasons for us discussing letterpress in such technical detail came out of technical discussions about electronic image sharpening — see last point.)

• The reason why both the correct impression depth and the precise amount of inking is so important has to do with optical and visual characteristics of the printed type. A correctly inked type block causes the ink to 'pool' or thicken along edges, in corners and in type serifs. As the ink is denser in these edge areas it's fractionally darker than ink in the more open areas of the type. This leads to an optical sharpening effect along the edges of the type which makes letterpress visually very crisp—much more so than offset lithography (which is renowned for soft 'edges'). What happens here is somewhat akin to the way say Photoshop's 'unsharp' mask works.

Here's the more detailed explanation for diehards (sorry, bits of it may be heavy going). In well printed letterpress, the optical sharpening effect that comes into play is essentially mathematically equivalent to transient/step response in signal processing—as in electronic sharpening in video and television signals where overshoot and white-to-black/black-to-white transients are involved. Here, reducing the transition risetime (i.e.: the shorter the time it takes for a transition to go from white-to-black the sharper that transition looks). This is the basis of Sine Squared Pulse and Bar testing in television.

For the mathematically inclined, here's the Wiki on Step Response: https://en.wikipedia.org/wiki/Step_response The three graphs in Figure 4 give a reasonable facsimile of the differences between offset lithography and letterpress. The first graph is representative of the not very sharp smeared black to white transition in offset lithography whereas the fast risetime and overshoot in the last graph is representative of the much sharper letterpress. For completeness sake here's the BBC monograph on Pulse and Bar testing (this was once one of my textbooks on the subject): https://www.bbc.co.uk/rd/publications/bbc_monograph_58