Deuce Traveler : You're familiar with the rat hierarchy in UU1. Good. Keep in mind that the spider hierarchy is the same, except that there's also a mid-tier spider known as the Wolf Spider. They're red. There's one on the first level somewhere. A lot of monsters have similar hierarchies, a good rule of thumb is, that the darker variety of a monster is more dangerous, but there are exceptions to the rule.
I'm gonna give you a few generic tips, hoping that I don't spoil anything:
# In your explorations you may have spotted a 'window' that peered out into darkness. You'll find more of them. Try checking them out once you're lower down in the Abyss.
# Flattery won't get you everywhere in the Abyss. What works for the goose, may not work for the gander.
# While brown worms (Rotworms) are pretty harmless, the green worms (Bloodworms) are pretty harmful. Be wary of them.
# The Banner of Cabirus (tattered tapestry with the Ankh sign) is a mark of civilized creatures. They hang out where the banners hang. Respect that.
# Lurkers (the green tentacle monsters in the water) are a great early source of XP, but they're also tough little bastards that won't go down without a fight. If they get the better of you, just avoid them as they're only dangerous when you're in the water or very close to it.
# That said, there are plenty of things to be discovered by swimming around, but if the screen flashes blue while you're swimming, it means you're losing health due to being a lousy swimmer.
# While lava doesn't instantly kill you on touch (like in Thief) it will burn you up eventually.
# In the case of both water and lava travel, 'alternate' means of transport are available.
# If your character build is magic-heavy, consider learning to do without it.
# Grey bats are harmless, the 'other' kind of bat is not.
# By now you should have a small collection of mantras to chant at shrines. The ones you find first are 'generic' Mantras that raise random skills, sort of. You'll find more specific mantras later on, so you might want to save up on those skill points.
# If you come across an 'upset spectre named Warren' (Get it?), leave him alone. He's a-door-able. ;-)