The United States is probably the most difficult country to collect, no matter how you figure difficulty. If your measure of difficulty is cost, then US stamps win hands down; a complete collection of US stamps, only done once, would be worth near $20 million, many times the cost of any other single country. If your definition of difficulty is how hard it is to identify different major catalog numbers from one another, classic US philately has the crown there, too.No other country has turned minor plate varieties into major numbers like the Scott catalog has done. On the 1851 issue (#5/16) even plate design differences (not printing differences) create major numbers. No other country has subtle shade differences resulting in so many major numbers as we have on the 1857 issue (#27/30A). Our Bank Note issues (#134/218) are a study in how to make a hobby off-putting, and the same subtle differences that make these postage stamps so difficult are duplicated in the various major number printing varieties of the Newspaper Stamps (PR numbers). Our Washington-Franklins are loaded with subtle types and design specks that Scott makes major catalog numbers and which for nearly every other country worldwide would only be minor varieties of the major number. And all of these are very expensive. There are more than fifty official governmental reissues of the Nineteenth Century made on the original plates in the original colors which can only be told by paper and impression (and often only by provenance) and which have major catalog number status. Our Official stamps have major varieties for paper types alone. Our Postage Dues have major numbers for shade types alone. There are over 100 stamp numbers that the Scott catalog lists as major numbers (and that there are spaces for in major stamp albums) that exist in quantities of less that a thousand, and which are all varieties of major stamp numbers that for any other country would be minor "a" number type varieties collected by only the most serious of specialists. And we haven't even talked about the Grills, which are a study in difficulty all their own.
Elevating minor varieties into major collectibles has done a great disservice to the long term health of our hobby. The reason that these stamps were made into major numbers in the first place was that the catalog in earlier times was controlled by professionals who had identified these rarer varieties, owned them, and had access to holdings of them. So they naturally promoted them. J. Walter Scott's catalog grew out of his price list, and, until about 1960, the listings were greatly influenced by dealers. And for dealers, the more varieties that are needed to fill an album page the better. But the long term impact has made earlier US philately too difficult. This is a hobby, not nuclear physics. By keeping our hobby so difficult we push away collectors who are unwilling to invest thousands of hours in study to discern varieties that make very little difference. Easing access to US philately would go a long way towards making it more popular.