I started the AD&D campaign last September. I wanted to use my old books and play the game I grew up with. I played it as close to the original rules as possible. I wanted to see how the old system held up and if I still liked it as much today.
First thing I learned is I much prefer ascending armor class. Eliminates the use of the matrix tables. It also gets a little confusing when you have some that are trying to use THACO. I've never played a game where THACO was used, but found out it does not match the matrix table in the 1st edition DMG. I've never played 2nd edition or ever bought the books. Having ascending armor class with 'to hit' bonuses for high levels is the way to go for me.
Second thing I found I didn't like were the five saves included in AD&D. There were so many situations where a save was called for, but did not fit into any of the categories. I made some approximations with what a situation may be comparable to, but it still was a stretch most times. I don't like the single save roll either. Where as I've never played 3.5 and only Pathfinder recently, the Will, Reflex and Fortitude save make the best sense to me. Those three options can cover almost any situation.
Third thing was I don't like a high magic campaign where magical items fall out of monsters butts when you kill them. Sure I could have toned it down, but I was trying to play like we played back in the late 70s, early 80s. There was a column on the character sheet just for magic items. While I did switch it up with the type of magic items. I used more unique items and sometimes the players discovered their powers and sometimes they didn't. I prefer a low magic campaign where if there is a magic item it has a name and history. Not number 4, +1 sword.
Forth thing was the monsters. I ran this campaign with lots of different critters and monsters, but I prefer the monsters to be like the magical items. A monster with a name and history. So when you do encounter a critter you better bring your lunch, its gonna be a fight. Not just a quick glance at the Monster Manual..."okay, I need to use my +2 axe to hit this one and has anyone got any silver?"
Fifth, I don't really like the experience point system all that much. I would prefer to streamline it and make all the classes need the same amount to get to the next level.
Now you would think with all these things I didn't enjoy myself, but I did. It was very cool to break out the DMG, PHB and MM and not just refer to them, but actually use them. Dialing up an old style AD&D campaign was a lot of fun. The players did an excellent job of keeping the story lines moving and solving problems. Would I do it again, hell yeah. But next time around I would do what any old school gamer would do, house rule it to fit what I like.