There is an experience bomb that can give you a bonus (see
History of the mysterious "Experience Bomb"). Some people have run into a bug with this which resulted in very high levels, but the average level at the end of the game is in the range of mid forties to low fifties.
A very low reputation may prevent you from getting or completing quests, if the people involved will not talk to you.
Just before hitting the gong, everybody except those in the council hall and Janus are expendable. With the latest
patch a few more of the required NPCs are protected, so you may not be able to break essential quests by going on a killing spree before this.
Swords are generally considered to be the best weapons; I didn't find many good axes after the catacombs. Your experience may differ, based on random stats. The game does not adjust your finds to where you put skill points, or change the weapon type or location of the swords consistently in the game, it is just biased toward swords. Historically swords tend to be the more popular weapons, so this is not entirely unjustified.
The dragon shield is in the castle armory, not one of the holy items. The holy sword is in the first level of Iona's dungeon, though.
VERY BIG SPOILER ON HOLY ITEMSVERY BIG SPOILER ON DRAGON ARMOR SET