It’s probably a mistake to think of the UN as a wholly separate entity from its members.
The UN is an assembly of sovereign states, and it acts as a mechanism through which the community of states can express and sometimes enforce its will provided there is a sufficiently strong consensus behind that will.
Take three examples:
As you rightly point out, there have been any number of General Assembly resolutions criticising various actions of Israel, but none of them have led to any enforcement action. GA resolutions generally aren’t binding or enforced; they may have some political or diplomatic value. Actual mandatory action is the preserve of the Security Council. That’s not to say that the GA has no important functions; it elects the rotating members of the Security Council, for one thing.
When Sadaam Hussein invaded Kuwait in 1991, the Security Council took prompt and ultimately effective action to remove him. This was, of course, led by the US, as was the military campaign which the SC authorised. The SC is structured so that it cannot act effectively unless all of the five permanent members are prepared to give action at least their grudging assent (and, in practice, effective action is unlikely unless one or more permanent members are enthusiastically behind it). This is deliberate; there needs to be a strong consensus, not just a simple majority of votes, behind any mandatory action. (A (very loose) comparison can be made with the US electoral college system for electing the President, which seeks to ensure that the winning candidate doesn’t just need an absolute majority of the popular vote; he also needs to have his support spread across the country.)
In 1993 the US completely failed to secure the necessary consensus in the Security Council for military action against Iraq. It went ahead and organised its own party with its own friends, of course, and there was – obviously – no consensus in the Security Council to prevent that. The rights and wrongs of the US position are not the point here (and, please, anybody wishing to engage with that issue, open another thread!); the point is simply to note what the SC, in practice, can and cannot do in various circumstances.
The bottom line is that, while there’s a great deal that the UN cannot do, there’s a great deal that it can do. But it can only take really effective action where there is a broad consensus in favour of action, involving the assent of all the five permanent powers and the enthusiastic support of at least one of them. Where those conditions exist, the UN can do a good deal. They rarely exist in favour of taking military action, but that’s not necessarily the fault of the UN.
The UN also provides a very useful forum for the development of consensus, for bargaining, and for enabling states or groups of states to arrive at common positions. Much of this is fairly low-key, so you don’t notice it. And much of it has nothing to do with taking military action. The international consensus that enabled smallpox to be eradicated, for instance, was formed through, and acted upon by, the UN. Similarly a host of international agreements on everything from the protection of the Antartic environment to the treatment of refugees have been developed, adopted and implemented through the UN. It’s slow, it’s cumbersome, it’s expensive and it hasn’t lived up to expectations, but it has been a more effective instrument for developing and implementing international co-operation than anything which went before, or any of the alternatives currently on offfer.