These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fanon/What Could Have Been: Though Word of God has never confirmed this, many fans believe that Turtledove originally planned to have the U.S. lose World War I and become an Expy of Nazi Germany, with Gordon McSweeney and Flora Hamburger becoming American versions of Adolf Hitler and Rosa Luxemburg, respectively. This would explain why McSweeney was set up as such a memorable character back in the first Great War book (even spending a short time as a viewpoint character) only to be unceremoniously killed in battle, and why Hamburger's name and personality bear such a conspicuous resemblance to Luxemburg's. It also makes the decision to put a military genius (known for his expertise with tanks) named Irving Morrell on the Northern side a little less baffling.
After the Great War, part of Texas is carved out and becomes a US state named Houston, despite being nowhere near the city. Sam Houston was not a supporter of the Confederacy and was in fact forced out of Governorship of the State after it seceded. Naming it Houston makes perfect sense.
The popular fan theory that Flora Hamburger was originally conceived as an American Expy of Rosa Luxemburg (rather than the AU version of Eleanor Roosevelt that she eventually became) actually makes perfect sense when you realize that even her name is an obvious derivation of Luxemburg's. Both of them have floral-themed first names (Flora/Rosa), and both of them have surnames derived from regions of Germany (Hamburg/Luxembourg).
Genius Bonus: The fishing vessel, F/V Ripple, that George Enos worked aboard actually was a part of the New England fishing fleet in real life, and was requisitioned by the U.S. Navy as a minesweeper during World War One.note Although it was requisitioned form the Imperial Russian government, which had purchased the vessel from its initial civilian owner and was unable to take delivery due to the 1917 revolution.
It Was His Sled: Now that Turtledove has published seven books detailing the rise of the Freedom Party and the Second World War, the revelation that Jake Featherston is an Expy of Hitler isn't nearly as shocking as it was early in the series. When the character was first introduced during The Great War, he appeared to be nothing more than a run-of-the-mill artilleryman who happened to hate Blacks a bit more than the average Southerner.
Magnificent Bastard: Clarence Potter, who managed to blow up Philadelphia with a nuclear weapon and get away clean. Potter himself seems to view Jake as one in-universe.
One could argue that Luther Bliss is one for the US side, especially given that he terrifies the above mentioned Clarence Potter.
Moral Event Horizon: The Population Reduction for everyone involved, but especially Jake and Jeff.
Boris Lavochkin's actions in Georgia and South Carolina.
The Mormon resistance actually comes off as pretty sympathetic, even obeying the Geneva Conventions, until they start using people bombs.
Protection from Editors: Harry has a really bad habit in repetition. It reaches a ridiculous new height in In at the Death when a chapter is duplicated.
Sequelitis: The series is eleven books long. It shows.
Some Anvils Need to Be Dropped: As Anvilicious and obvious as the parallels between the Confederate Population Reduction and our world's Holocaust is, it does enable Turtledove to make some important points about how such atrocities happen, about the bureaucracy involved and how most real-world Nazis and war criminals are not Card Carrying Villains that are easier and more palatable to digest in fictional form.
It's worth pointing out that the Freedom Party is actually a little believable - the CSA did unquestionably get stabbed in the back and the rebellion simmered off and on for two decades after the initial revolt. It's certainty more believable than Hitler, who really just hated Jews for no good reason.
Also more realistic in that unlike the South, which had always had ample dislike for blacks as a race, up until the 20th century Germany historically was one of the safest places in Europe to be a Jew (Frederick the Great was the first major European leader to enact complete religious freedom in his nation). Reality Is Unrealistic, indeed...
In a lot of ways the Freedom Party is similar to the Real Life Ku Klux Klan in espousing traditional white Southern values and protecting that way of life from encroachment by blacks and foreigners by any means necessary. The mystical trappings are gone, but if Featherston had gone the full Hitler route in establishing a new mythology the Klan as we know it would fit well within that scheme. One can think of the Freedom Party as "What would the KKK be like if the Confederates had won?"
Also: racism is bad. Period. And the South winning the Civil War would not, despite the cries of many Confederate diehards, have been a good thing.
Tear Jerker: The Population Reduction, especially the deaths of prominent characters like Scipio and Hipolito Rodriguez's suicide. I even cried when Jefferson Pinkard's family visited him before his execution. Much as he deserved what he got, there was real love there.