We'd enjoy a lot more freedom than we do now and security in airports and major events would be much more casual.
Less western presence in the middle east.
Iraq and Afghanistan wouldn't have been invaded without 9/11 justification.
Less mistrust of Muslims.
9/11 was perfect fodder for Xenophobes.
Less militant and aggressive atheism.
Atheism was obviously around prior to 9/11 but most agnostic people viewed religion as harmless or even generally positive, at worst they felt it was a mild irritation when religious people knocked at the door. 9/11 made people realise that religion didn't just disturb you in the shower, it could fly an aircraft into a building. Suddenly many saw religion as dangerous and atheism became more organised, militant and widespread.
A different political landscape.
National Security would be less of an issue in elections and campaigning in most Western Countries.
It should be pointed out that if 9/11 didn't occur then it's highly likely that a 9/11 like event would have occurred soon afterwards. If the plan to attack the Twin Towers and other buildings had all fallen in a heap in the planning stages, another similar event would have occurred not long afterwards. The world wouldn't be the same as it was in 2000.