Pardon me for my lateness, but I don't think the TB was so far discussed, especially in the light of recent developments in other books and the whole re-re-re-reboot thing. And since I got it today, and it made me reread the whole story at once, here are some of my comments and speculations:
The Extras
1) 2 pages of Gary Frank's costume designs with some notes for Johns. This kind of kills the idea that this is not the same Legion from before Crisis - from the notes, the alterations in costumes Gary seemed to take were purely aesthetical.
2) An introduction by Keith Giffen, who basically write an eulogy on Johns saying he is just as much a creator as Shooter, Levitz, Waid and DnA were to the Legion (he said he was dragged along by Levitz, in his self-deprecating style). He said that when a creator touches Legion, he wants to go back and so he expects to see Johns back soon. Say whatever you want, but Keith is probably the biggest Legion fan among creators.
3) All extra covers. I particularly enjoyed the Grell and Giffen one (in another different style from the one he used last on Legion 1,000,000). Gary's last cover, with Superman recharging in the yellow sun, is gorgeous.
Speculations - Spoilers?
1) Brainiac 5 keeps mentioning he is tampering with the Speed Force and that he needs a certain character to make his experiment work. I'd say it is Barry Allen or at least a speedster. That would be XS - according to his recent interviews.
2) Someone has planted the information on Superman on the Arctic - the information Earth-Man used to create his Justice League. This plot was not solved so far. However, one dialogue seems to hint that this was planted at the 21st Century in order to make it happen. This sounds very much like Superboy-Prime material, and seems like a logical step to link this to L3W.
3) Ah, the Subs. This is my favorite LSH concept of all - right along with real-time stories and chronology (both dead since TMK). That was the most refreshing bit on the whole plot and, at the same time, the saddest. Refreshing because Johns maintained the whole humourous vibe set by Levitz and Giffen without making them punchbags. I particularly enjoyed the notion they were not accepted for lack of experience but not for their morals. This is very good to read these days.
What I didn't like was that the disappear for no reason after a couple of scenes with the Justice League of Earth. Moreover, when Superman draws the Legion at the last scene, there isn't a single Sub around, as if they didn't matter. Shame on you, Johns.
4) Remember there were 2 Green Lanterns in the 31st Century, with one being Rond Vidar, according to Geoff Johns? Nice. Then, read Rainbow Girl mini-bio:
Wields the powers of the mysterious emotional spectrum, resulting in unpredictable mood swings.
First, this is a retcon: she did command the power of colors but emotional spectrum is a Green Lantern concept from Sinestro Corps.
Better yet, when she uses blue, she says she is using the "power of hope". When red, the power of rage (angry Rainbow Girl). And when she turns green, surprise! The power of "will". Take my words here: Rainbow Girl will be a key character in Legion of 3 Worlds.