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.
Broken Base: The British in Tales of Valor. 'Roo rushes. Don't dare mention if they are overpowered on the Relic forums, because you will learn what it's like to be on the receiving end of a Firestorm barrage.
The second game's... skewed ideas about how faction balancing should work. There was a string of patches where Soviet vehicles were vastly underpowered compared to German ones. It's gotten much better, and this includes the newly added American and Overkommando West factions.
For Allied players, Puma armored cars. The bastards are damned fast, virtually immune to small arms, and will kite your anti-tank infantry from beyond effective retaliation range with their 20mm cannon—and if you send M8 Greyhounds (the accepted counter) against them, they have the nasty tendency to pop in a 50mm gun to serve as tank destroyers on the cheap. The saving grace is that players who rush for a fast Puma generally can't get heavier tanks out very quickly; it also usually means that they've neglected their Tier-II units, particularly the heavy infantry and anti-tank guns.
Anti-tank guns themselves count for anyone who tries a tank rush: it is incredibly frustrating to watch your expensive tanks get blasted apart at range by a cheap little 57mm/Pak 38. This is fully intentional, by the way: tanks need infantry to back them up and to clear AT nests. German players relying on either heavy tanks or a lot of tanks are especially crippled by AT guns, as they tend to not have much, if any, infantry due to how expensive their tanks are.
Do you like tanks? Well, U.S. Rangers don't, and they're here to crush the hopes, dreams and fun of German players. What makes them so dangerous compared to Airborne is that they don't have to pay for their bazookas; instead, they come with the unit. They're also ridiculously cost-effective, and are capable of tearing through most infantry at close range if you upgrade them with SMGs.
Mines are, of course, invisible to your troops and require a minesweeper to detect and remove, which is pretty annoying as it is and likely to quickly induce paranoia. But it is when your enemy manages to place them with a machine-gun nest covering them in a chokepoint that you'll be really irritated by them.
Most Wonderful Sound: Your commander informing you that you're now able to use one of your later-game doctrine abilities after unlocking them.
Your heavy tanks' commanders' voice lines tend to imply they enjoy the obvious power of being in one. That'll probably rub off on you as well.
Hearing your troops report they've wiped out an enemy squad or destroyed a vehicle.
Listening to your troops report they've got their ass-kicking weapon upgrade.
Scrappy Mechanic: The player's troops can be stupid when it comes to enemy vehicles, with a tendency to run up to the enemy vehicle and get themselves killed. Even if a soldier so much as touches the enemy tank there's a good chance they'll instantly be killed. Yes, this can include running into the side of a stationary vehicle. This can wipe out 4 or 5 men from a single squad in under a second. Worse yet, this seems to be highly inconsistent - at times, the men decide they like living and jump out of the way of enemy vehicles rolling by, other times they get effortlessly run over.