Tough question this time... WHY do you love the Lone Wolf series?
I think it was the maps that did me in. When I first started reading, I was probably about 9 or 10. I was reading several gamebook series alongside Magnamund: Choose Your Own Adventure, the Real Life series, Fighting Fantasy, as well as the Storytrails series.
What struck me regarding Lone Wolf I think, was that it wasn't scattershot like how Choose Your Own Adventure and Fighting Fantasy often had books that intertwined with one another, but there weren't enough of them (ie Choose Your Own Adventure's Harlowe Thrombey storyline covered by "Who Killed Harlowe Thrombey?" and "Ghost Hunter").
Lone Wolf was different. It had lots and lots of good illustrations, the artwork for the maps (until Bk 9) was beautiful, and additionally, I think Joe Dever (bless him wherever he's gone) also knew wisely to concentrate on one character and one character (with his environment) alone. I often cheated, skipping combat and Random Number sections, just in order to explore more of the world he created, and to see more of the beautiful illustrations that Gary Chalk made. Even to this very day, when I ask random people who have never heard of Lone Wolf (or even gamebooks for that matter) for their opinion on artwork, the one work they often pick which looks best for Dever's books is Chalk's.
"Doubt not your purpose nor your strength, for inside you there burns a flame that can light the hopes of future men for all time."
Gary Chalk's artwork is definiely distinctive, and definitely is one of the things that solidified my love of the first books. The world is also well developed, reading the Magnamund companion with the histories just shows how rich it is.
A typical problem, limitation really, of solo gamebooks (regardless of nature) is they tend to be very two-dimensional. The time-consuming challenges of designing them structurally, laying them out, closing off issues, playtesting the game mechanics (for gamebooks that have them), editing, and publishing them...all those challenges add up. And cost to produce translates to cost to purchase (or, more commonly, cost to purchase puts a brutally solid limitation on what can be spent to produce). That being the case, actual creative writing, particularly of in-depth text, gets pushed to the very bottom of the priorities. And one sees that in the relative superficiality of most solo gamebooks and series.
This is, actually, still a very modern problem with designing and producing computer roleplaying-style games. I have a few author friends who have, over the years, been pulled into the "content writers" role in building computer roleplaying games (both single-player and MMORGs), and they all say the same. There's never really time or budget or final understanding of the game provided sufficiently in advance of final production and release to really succeed with in-depth story or significant world building beyond the minimum necessary for the central purpose of the game.
Somehow, through all the limitations, Joe Dever was able to beat the norms with Lone Wolf. Even though he necessarily had to focus his worldbuilding in each book to that which was relevant for each book, he still found the time to add much that others struggle to develop. He also managed to sustain story and world continuity through the books, often to such extent that it's a rare and genuine surprise to come across a contradiction. Somehow, he managed it, and yet these books were hardly expensive (even for book prices of the 80's and 90's). It's always impressed me how well Lone Wolf somewhat apart from nearly all of its contemporaries in this, a fashion that particularly resonates with me (as a reader and gamer a like, I have a substantial appetite for story development and details; most fall short of what I really want to discover and learn). And, years later, his work continues to stand up well with other game productions (book, computer, console, etc.) that literally have teams building them.
I could list a hundred other response to your question. But the above is probably the most fitting.