I think the Marvel movies, and also the possibility of future cartoons (F4 is on the way) is a big reason. We all know that the toy companies say that kids drive the sales, and in these types of cases, they do. We see it in Star Wars as well. In non-movie years, I hardly ever see kids looking at Star Wars stuff...but in movie years, especially last year, for the first time I saw kids more than I saw collectors. I still see kids looking this year, but I think that will slowly taper off as the movie gets further away. I personally don't have any kids, but going by our nephews, cousins, and just kids in the aisles...I don't know that there are many (or any) action figure lines that kids are interested in at all that aren't tied to either a movie or cartoon show. I think, like Jeff said, that's what Hasbro is looking at. Spidey, F4, X-Men, Hulk, etc. could or will have movies and/or toons coming out over the next few years, and that's when the kids are in the aisles in full force.
That's one thing I've noticed with Marvel Legends, I can honestly say I don't think I've ever seen any kids looking at those in the store, let alone buying any. The classics lines, particularly Spider-Man, yeah..sometimes. But, when the movies are out, its a big draw to the "movie lines". I saw kids looking at the Spidey stuff all the time, and even with Fantastic Four, which wasn't quite as much of a "big deal" movie wise got a lot of attention from the kiddos, at least around here. Not on the level of Spider-Man, but more than Marvel Legends sees. I guess, overall, even though I wish it was different...kids just aren't interested in comics anymore, unless it is in movie or toon form. Its too bad, because I think many of us grew up reading comics, but its more of a video game/electronics generation now. Heck, I don't think I even had a TV in my room growing up until 6th or 7th grade, and even then it was an old "junker" that was used for Nintendo only basically. Now I have cousins/nephews/friends' kids who have them at 4 years old or so - sometimes with Xbox/PS2 level gaming systems, DVD players, or even cable/satellite. Crazy.
That being said, its tough to see where things will head. I could see a scale change, or at the very least a line revamp. With a few exceptions, the Legends line is really getting into lesser known territories character-wise, and I can't see Hasbro launching their first few waves without a basic Spider-Man, Wolverine, Hulk, Iron Man, Cap, etc. leading the way. Or, they could change things entirely and focus in on movies and cartoons. I know the press release announcing the Marvel/Hasbro deal seemed to zero in on Spider-Man 3 (and beyond) as well as the Ghost Rider movies. It'll be interesting to see what happens.
On a total un-related note, but I something I forgot to bring up earlier in this thread. I don't know if anyone else has noticed this or not, but the SuperHero Showdown/Marvel Showdown figures have really seemed to be selling well around here. I haven't bought anymore myself, but I know the pegs at Target are bought down to empty fairly regularly and WM is often the same. Maybe its just my area, but this line seems to be taking off more that it is available in the other retailers. Just an observation.