Another vote here for level titles, neutral assassins, rangers with magic-user spells, effective multiclassing spellcasters, and a DMG that included a treasure trove of tables, miscellaneous info, and reading recommendations.
My biggest vote goes for the idea of balance, however. (/rant on) In 1e, Gary Gygax didn't believe in the idea of balance as we know it today. He said right in the PHB that the magic-user and the monk may be the most devastating classes at higher levels. I miss having unbalanced character classes. The weak-at-low-levels, strong-at-high-levels magic-user and the reverse-designed fighter made a lot of sense to me. I don't think it's possible to perfectly balance every character class at every level, and I think it's up to the campaign and the DM to make sure that the game is fun for everyone. If someone doesn't want to play a class because it's not 'powerful' enough, then the DM should adjust his campaign accordingly. It's all about the players. Having unbalanced classes gave them more options. They could choose, by their class, to play in a low-powered or high-powered campaign - as long ast he DM adequately adjust the campaign to keep the game fun for the players. Balance as we know it today is a misguided port to D&D from computer games. In computer games, the computer can't adjust the campaign world on-the-fly to the choices of the player, so in a computer game, the designers have to make sure that all the class choices are equally fun and balanced to play. But in a pen-and-paper world, the DM can adjust the game for each individual character, even in a group - and she should. Given that DM adjustment, the inclusion of unbalanced classes gives players more options. Namely, they can choose to play in a high-powered or low-powered game by their choices.(/rant off)
Still, there are many more things that I don't miss.