Because it seems there’s a lot of information people just don’t know! I've also added a Glossary of Term. Please feel free to add more!

# No ladder
# No monthly fees
# Separate Gateways (Similar to D2) with separate datasets (separate stashes, characters, gold, artisans’ etc.) These would be D3 equivalents of USwest, USEast, Asia, Oceana etc.
# Four difficulties
# Level target for Normal: 1-30
# Level target for Nightmare: 30-50
# Level target for Hell: 50-60
# Level requirement for Inferno: 60
# Inferno monsters start at level 61.
# HC mode is unlocked upon reaching level 10 in SC
# Followers are unavailable in multiplayer games
# Best source of loot = Random Champion packs, not bosses (potential exception once Nephalem Valor details are finalized)
# Most unique items get a few random stats like Rares
# Some Unique and set items will be craftable from Artisans
# Gems don’t have level requirements (you can use the best gems on level 1 characters)
# PvP will not exist on launch, it will be implemented post-launch
# Skills are made with what are called “snows”, which are different data sets between PvP and PvE. This lets Blizzard give skills different stats in PvP than in PvE.
# Diablo 3 is online only
# Party cap is 4 players
# The level cap is 60
# Players are intended to be about level 60 by the time they finish Hell difficulty
# RMAH is softcore mode only
# No Charm system (which was the Talisman which has been removed, likely an expansion feature)
# The entire game is soloable
# Permanent HC deaths in PvP is currently undetermined
# PvP structure/format is not finalized, currently it’s a “team death match” format
# PvP is currently 4v4 (as per the Blizzcon 2011 Demo)
# Modding is not supported
# No Skill trees
# Stats are now automatically assigned upon level up
# MF is averaged between the entire party
# Loot drops are private (you only see your own loot)
# Party size increases monster difficulty (Seemingly just HP in the Beta)
# Party size does NOT increase number of drops
# Party size does NOT increase the quality of drops
# Party size does NOT increase amount of experience gained
# Skills are restricted to certain keys by default. This restriction can be removed by choosing elective mode (in the options)
# There's an end game MF buff mechanic (called Nephalem Valor) for level 60 characters that increases your MF for every champion you kill with an undetermined cap.
# All skill damage (spells, abilities and attacks) is based off weapon damage
# Shrines now effect all party members within range
# Stamina has been removed
# Walking has been removed (We only run now)
# Potions now have a cooldown
# Scrolls of Identification have been removed (Simply right click on Unidentified items to ID them)
# Scrolls of Town Portal have been removed (You get a permanent, unlimited use TP item quest reward very early on)
# Monks and Barbs by default have 30% damage reduction
# Your stash, gold and Artisans are shared between all your characters of the same type Mode/Region
# The hostility mechanic has been removed

Glossary of Terms

# HC = Hardcore
# SC = Softcore
# RMAH = Real Money Auction House
# GAH = Gold Auction House
# D3 = Diablo 3
# SK = Skeleton King
# WP = Waypoint
# TP = Town Portal
# NV = Nephalem Valor
# MF = Magic Find
# GF = Gold Find

Source: http://us.battle.net/d3/en/forum/topic/4496519674?page=1