I had a lot of trouble being able to really recognize what parts of the book are new material and which are just directly the original Traveller Rules. So what do we get in Mongoose Traveller? Well, for starters, the whole system is really very familiar to me. The things that weren't particularly interesting to me were overly complex mechanics on things like starship creation or space combat that just wasn't my thing. And yes, I loved the rules for creating systems and planets. Games could be very high-mortality or (like in the case of my last T20 campaign) could run for years without a single fight (there's not many old-school games where that's a common occurrence!). A Trav game could be very military, or not at all it could focus on exploration, or on trade, or be set entirely on some world (be it a distant frontier, or a crucial planet in the heart of civilized space).
Now, on a purely personal level, I can tell you that what I loved about Traveller was: its character creation rules (which for an old-school game were truly amazing in how they created a whole backstory for your character through a series of rolls and choices), the simplicity of play, and the variety of campaign themes you could run with.
So in a way, I guess I would be exactly the kind of person the various purveyors of new Trav editions would like to get as a customer. More recently, I had bought the T20 Traveller rules, which I quite liked, and ran a spectacular campaign for about 3 years with it. In my roleplaying infancy I had bought the basic Traveller boxed set and later several other Traveller books, and I ran quite a lot of games with it. I would not call myself a Traveller fanatic, but I do have very fond memories of this game. ( Maybe the most elegant cover-design in the entire history of roleplaying, and it was invented in the late 70s)