I can't contrast 2e and 3e, but I can contrast 2e and 4e. Taking for instance the 2e Complete Fighter's Handbook, its a fairly nice book, with a good bit of fluff. Probably half the material is flavor text, but its a bit hard to say for sure because there are a bunch of things that aren't clearly one or the other. Because AD&D tended to have a lot of very fuzzy and story-based rules you can interpret a lot of stuff as kind of both or consider it suggestive but factored into the design of a kit. A lot of the crunch is also written in a very conversational style and mixed into the text.
Honestly I liked the 4e style better. If you look for instance at Martial Power, which is very analogous in 4e to CFH it has many similarities but is definitely stylistically quite different. It covers all the Martial power source, so its less focused on the fighter specifically (though about 1/3 of the stuff is for fighters). There is more flavor than people give it credit for, but the division between crunch/flavor is pretty hard. The result is its both easy to ignore the flavor and easy to focus only on the mechanical stuff. However, MP sets up the explanation for each new build pretty carefully. What it doesn't do is say "this battlerager is a viking!" He's drawn to LOOK like a viking and his description SOUNDS like a viking berserk, but its never stated outright. I think maybe this is part of 4e's issue. It lets you draw a picture and build a story around it instead of coloring it all in. For some people this is more fun, it pokes the imagination but doesn't lead you by the nose to one answer, but I guess for a lot of people it felt flat. PERSONALLY I liked a lot of the 2e writing, but I thought it was overly heavy-handed in a lot of places. 4e's touch is often lighter.
Again, it will be interesting to see what 5e does. When they started this process it was hard to imagine they could escape from what they unleashed with 4e, but Mike has shown great determination to remake AD&D, so my guess is the fluff will be much more in the leading you by the hand mode.