The NRA contributes large sums to get right-wingers elected, right-wingers refuse to hear event the most rudimentary of gun control laws; Quid-pro-quo.
That's not really the same situation as this. Gun-rights candidates (and gun-control candidates) announce their position before getting contributions and before getting elected. Their positions don't change from before the contribution to after.
Here what we have is Hillary Clinton refusing a meeting (with the Crown Prince of Bahrain) and then allowing the meeting...after he makes commitments to the foundation. I.e. we can actually see a change in her position before and after the contribution. That's a real quo. Something changed.
Note that separately both actions are fine and legal separately. But here we have proof that Huma Abedin (Clinton's direct assistant) knew about the contribution. Clinton had promised not to have anything to do with the foundation while Secretary of State. She clearly broke that promise.
Another issue is how the Crown Prince of Bahrain saw things. Perhaps he felt extorted by the foundation. As people have been pointing out, there was absolutely nothing wrong with granting him a meeting. The problem is that he was kept off the schedule, paid the Clinton Foundation, and was then put on the schedule. The claim is that he was put on the schedule due to a new opening. He could easily feel ill-used here, as he had reasonable cause to expect a meeting to be granted without bribes.
The final issue is that campaign contributions can't be used outside the campaign. If a candidate has extra money at the end, they can either carry it forward to next time or donate it to someone else's campaign. They can't just spend it or put it in their own bank account. The foundation could give money to the Clintons (although they claim it hasn't so far). They could pay themselves salaries and reimburse expenses.
Personally, I have a problem with campaign contributions given to incumbents. I don't think that that should be legal. If someone wants to run for office, they should give up their existing job first. I had that problem when Clinton, Barack Obama, and John McCain were running. They should not have been able to run for president while in office. Further, once in office, presidents (and other office holders) should not be able to attend fundraisers in my opinion. But the law's not like that in the US currently. Campaign contributions are treated as a special case. Unless you can prove that a candidate something because of the campaign contribution, it's not illegal. And that's ridiculously hard to prove.