With respect, what does it matter what OBL "biggest" grudge is, or was. He hates the west, as does his followers, and would have no shortage or reasons to draw on. Only he knows what his biggest gripe is, and at this point, who cares.
Secondly, I would caution people who talk of the Cold War with a sense of nostalga. Yes, its true the US and NATO had stronger militaries, but then we were facing a very large military force in the USSR and the Warsaw Pact. It cost a lot of $$$, and the chance for an accidental nuclear exchange was very real.
Look, we ended up with getting the rest of Eastern Europe out of Soviet domination, as well as removing any immediate threat to Western Europe, without having to go through a costly war to do it. Thats a good thing. Ex Soviet client states ran out of money, so they had to get on with getting their economic houses in order, thats a good thing. Now they want to trade, not build more munitions (there are a few holdouts, like N. Korea, but not many).
The Middle East has been a problem for many years, it was a problem during the cold war, and its a problem now, but just because there are issues with Islamic Fundamentalism isnt cause for seeing the old West/East stand off as "the good old days". That is a "glass is half empty" view imho.
As for a bi-polar world, I would argue we still have that. We have states that want to trade and live in peace, or are moving towards that goal, and we have either failed states, or states that have populations or governments that really dont want peace (at least not with us), that need to be dealt with by the world community. Hopefully without war, but we have to have that option if and when its required.
Terrorism, either domestic or foreign (in its many variations) will always be with us, there is always going to be some splinter group somewhere that has enough people who are willing to use violence to their ends, they too need to be dealt with.