Here's the quickest lesson;
29; Had certain quest rewards level 35+, TBC Enchants used to require an item of item level 35* or higher, so people would get these quest rewards, put an extremely overpowered enchant on it (unaffordable for a lot of twinks, mainly because the mats cost was extremely high in the first place) and literally solo battlegrounds.
39: The above is the main reason 39 was sometimes even prefered over 29, you got the most powerful enchants in the game, on the lowest level at the end of a bracket (29 never got you a full set of item level 35+* items). This is why I initially went from 29 to 39 during TBC. You were killing level 60s easily, sometimes being able to kill fresh 70s as well. Sure you could also do this on level 49, but it would be less impressive. Scarlet set as well.
49 is probably the most niche bracket I think. It had more abilities but probably most importantly for twinks, this was where the epics started dropping. Played it for a while, but not too long. Still have my shaman there now though. That said I never had any special appeal to this bracket, though you had some impressive epics here.
50-59: Raiding, arch. items., socketed gear at 59.
* = I'm saying item level, but it was just counted as weapon/armor level then and only quest rewards could have a higher weapon level than any drops.