Having re-read the book and thought about the rules, some advantages of second edition over other D&D (even B/X!) and in some regards (proficiencies, fighter rules, spells) first edition:
- Ability score bonuses are pretty small (+1/+2 at like 18) and start at higher scores (16+ usually), which means that they are overall less important to the game than in B/X. A character does not suck in comparison to his friends and the NPCs simply because his fighter only has 11 STR compared to 16 or something, because he's only missing out on a small bonus. And the way scores are generated, 4d6 drop lowest 1 die, increases the likelihood that the PC will have one great score to feel important. Other stuff the ability scores do is simply provide a guide to the character's basic abilities, like how many languages known or their max bench press. This allows them to be used as a guide for in-game decision making
- The class abilities are well-done. I especially like the fighters in this edition. The full impact of the rule is somewhat buried, but basically fighters are the only class that can specialize in weapons and doing so gives 2 attacks every other round starting at 1st level! This is an awesome ability that really makes the Fighter shine (everyone likes multiple rolls!) and is also simple and straightforward. No counting HD of enemies or something. This ability being denied to Paladins and Rangers also makes Fighters actually more important, which I like as I think the latter classes should be available.
- Class requirements, in both ability scores and race. This helps keep things thematic and balanced. Also, the classes being organized into groups of Warrior/Priest/Wizard/Rogue helps if you want to make custom classes that still fit in the mold without breaking the game, and keeps similar classes together in terms of HP. This level of rationalization of the rules helps.
- There's lots of options for weapons, armor, and spells, which are the meat of any game. Weapons come in all varieties, they list initiative modifiers (but this is an optional rule!), you can turn on/off weapon type v. armor type, etc. All the spells list their "School" or "Sphere" for wizards and priests respectively, so you can easily create custom lists to fit your setting.
- Spells are not under-powered at low levels. A specialist wizard gains a few more and then the spells no longer do paltry damage amounts. Instead, its reasonable. Also, it takes soooo many level of wizard to get the truly great, gamebreaking spells, so this isn't such a game breaker.
- Proficiencies are actually awesome, especially non-weapon proficiencies. Now some may turn up their nose, saying they're too close to a skill system, but I in fact feel the opposite. They provide specific "extra" (though mundane) abilities to characters to let the player and DM know what the character can do. Does my character know how to ride a horse, yes or no? If so, you'll not need to make a check - so in actuality reducing silly "can I make a check" to do everything under the sun that comes up in a game. The DM can simply look to the character's proficiencies to know whether they know about some topic. The way I deal with them though is to leave the "slots" available as the character plays - if in play Heraldry comes up as something useful, and it's available to the character class, I let the player decide whether they want to spend one of the limited (3/4) slots to gain that knowledge. Then going forward she'll always know about heraldry. It gives some structure to making the character unique based on in-game play.
Beyond the specifics, generally what appeals to me about 2e is that everything is very thematic and it shows the DM a roadmap for making the rules fit their own particular setting. Want to run an ancient greece game? Swap out this and that, deny certain classes and races, use the bronze plate rules but drop regular platemail, etc. Want your magic-users to only have illusion and divination spells? You're ready to go because all the spells are divided into schools. The core classes like Fighter and Priest fit many settings because their abilities are so straight forward (extra good at weapon attacks, being restricted to particualr deity's spheres of influence). And its faster than needing to come up with all the customized rules for your setting, because 2e's rules are such a grab bag of goodies.
That said, of course there are some major flaws, especially in light of the way we've all being playing D&D in the many years since this was published.
- Information organization and presentation is somewhat suck. There's no handy dandy bullet point list of class abilities or charts with all the options. you have to read through each class and race description. Rules are spread out in separate chapters (e.g. weapon damage is one place, number of attacks with weapons another, the effects of specialization a third). This can be solved with a simple reference chart you make yourself, but ugh, why make people do that work. At least the authors put a compiled set of most of the game's charts in the back (handy spells by level and spells by school/sphere are there too!).
- The weapon damage rules are dumb. They make no sense in terms of why would I pick a horseman's flail over a "polearm" over a halberd. Some are doing like 1d4+1 damage but seem substantially the same weapon as another doing 1d10 damage. This leads to hidden 'traps' for unwary casual players - ooohh I want a Ranseur! Well, johnny that does less damage than a battle axe but requires two hands and basically it sucks, you fail. I looked to Astonishing Swordsmen and Sorcerers of Hyperborea and yanked those rules. (The weapon damage problem goes back to AD&D 1e's also nonsensical damages etc.).
- Dexterity is overpowered because it breaks the pattern I noted above that scores only give small bonuses. Not only does it influence initiative, chance of surprise, ranged attack, and AC, it gives BIG AC bonuses. Nerfed when I stole more rules from Astonishing Swordsmen (AS&SH, pronounced Ash).
All in all, I highly recommend giving second edition another look. It overall is not more complicated than B/X but has alot more options and food for thought. So if you're like me and find it easier to pick and choose from a menu of rules and ability options rather than create them from a skeletal baseline, get yourself a copy and start playing AD&D again.