Huey Long was more scary to FDR within his own party than for governing generally. FDR understood that Long threatened his own base, which, if taken, would empower Republicans. A Long candidacy in '36 would have been damned more interesting than what it was without him. If he had lived to make the run he would have been a far larger threat to FDR than to Landon.
The old line that "Franklin Roosevelt saved the US from violent revolution or dictatorship" is one that people can't toss around as easily today as they did in previous years. But the danger to us is that we have become too complacent and assume that such things can't happen here. Things weren't fated to end up as they have. They could have turned out much worse.
FDR's significance may have been more international in the end than domestic. It may be that Americans don't go in for revolution, dictatorship, or socialism, but that hasn't been true of much of the rest of the world. Roosevelt might have pursued a policy more akin to that of Britain in the 1930s with better economic results.
But British leaders -- McDonald, Baldwin, Chamberlain -- weren't especially charismatic figures on the international scene and couldn't match the appeal of Hitler, Mussolini, and Stalin to many. Even Churchill's appeal was limited by the continent's dislike for British Toryism and imperialism.
We needed much wider support to rebuild Europe and Asia and make a common front against the Soviets. FDR's attractiveness to foreigners had a lot to do with what we were able to achieve later on, and it was precisely some of the things that American conservatives deplored about the New Deal that made many Europeans and Asians look to America with hope.
So the West may have been fortunate in having a figure like Roosevelt who had an attraction across a wider spectrum of opinion. It may have been a cheap magic, and it didn't always work, but it did have its successes.
The other thing is that America had its 20th century "revolution" in the New Deal, and a rather moderate revolution it was. We went through it and saw its weaknesses and failings. So we weren't inclined to major changes after the war. That wasn't true of Britain and much of the rest of the world. They were still hungry for fundamental changes after WWII, and they got them.
I doubt many people are going to agree with the last generation of liberal historians that FDR was our greatest president. But simply because we got through our greatest Depression and the world's most important war with Roosevelt in the White House, there's a limit to how far FDR's stock can fall.
Googling around, though, I found out that in 1942 FDR ordered a 100% marginal tax rate on income over 25,000 dollars. Every dollar you earned above that would go directly to the federal government. Of course, $25,000 was a lot more money then. And there was a war of great significance going on. The tax didn't last. Congress settled for a 90% top bracket. It stayed in effect for some time after the war (though there were those loopholes). But it's a good indication why liberals love Roosevelt and conservatives don't, and why it won't do to get too sentimental about the man.