I disagree, the current system, although not hugely realistic, is far more so than the skill unlock concept that people keep bringing up.
The current system assumes either that everyone is born with all the knowledge they need to become legendary on their own, or that everyone is ingenious enough to figure it all out through trial and error. That is not realistic. That is the gap in realism that this idea seeks to fill.
Yes someone has to learn how to do something but people can learn by doing, there has yet to be a task suggested that this isn't true for. Remember we are not inventing only imitating here.
Firstly, keep in mind that the strict limitation is only meant to apply to those tasks for which it makes sense. It wouldn't affect all skills across the board.
Secondly, there have been such tasks brought up. Consider NW_Kohaku's mention of chemistry-related tasks.
Other tasks, which have been developed and refined through generations, may be doable (very poorly) without acquiring that knowledge, but only with some form of instruction would you learn how to do it well - unless you're an exceptional genius.
Another example that hasn't been mentioned yet: Building/designing a loom. If your fortress has no loom and nobody knows anything about weaving, assigning someone the Weaving labour and ordering a loom workshop to be constructed, you're going to need a source of knowledge to get it built unless you plan to reinvent the thing.
To be perfectly honest, it's much, much easier to manage to make something than people seem to think. I made a usable bow in my backyard when I was six, with no knowledge of them other than having seen them on TV. It was weak, and I never made any decent arrows, but it worked. Had I had an actual bow to copy, it would have been far better.
Bows are also very simple. I did the same thing when I was a kid. This isn't representative of all tasks in Dwarf Fortress. Also keep in mind you are probably smarter than the average person/dwarf if that's the kind of thing you did as a kid.
Now, if you continued to experiment with making bows, without any instruction or source of knowledge on making bows, would you eventually succeed at making bows equal in quality to the best bows of the middle ages? Unlikely. Apply this same thinking to things that are more complicated to make than bows, and the likeliness becomes even more remote. This remote possibility is pretty well covered by the possibility of making an artifact.
You [and, as I saw reading further, Unfrozen Caveman] do bring up another possible source of training: More clever dwarves might be able to learn a crafting-related skill through reverse-engineering from a crafted item, much like using a book.
The point is, there's no invention in DF, and only the magic mechanisms are particulary complex.
This isn't just about the physical complexity of a completed object. This is about the methods used to create things and perform tasks and the knowledge required to do so.
Still, I would had to see the game lose it's "learn anything by doing" aspect. I can deal with not being able to make or do certain things without a certain skill level, but there should still be something my useless peasant can do to learn the skill.
Get training from another dwarf, learn from a book, or learn a skill that doesn't require training. This system really doesn't cause any problems there.