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.
Awesome Music: In sharp contrast to the laid-back soundtracks of the other games, the electronic version of Legendary Asia has a percussion-heavy soundtrack befitting a Himalayan adventure.
Crack Is Cheaper: The base games (USA and Europe) are a daunting $50 right off the bat. Ouch. Thankfully, the card expansions and bonus boards are much more affordable.
Epic Fail: In the online version, players who quit are replaced by "bots" that are generally much easier to beat than real live people. Losing to something named "Dumbot1" can be a bit of a blow to the ego.
Game Breaker: Some feel this way about new rules introduced in the various expansions, while others feel this way about the original USA game. Some examples:
Drawing two coast-to-coast tickets with the endpoints of both conveniently near each other at the start of the USA game can give a player an unfair advantage. As an example, if you can complete Seattle-New York, you only need two more segments to complete Montreal-Vancouver, which will net you over 40 points.
The 1910 expansion introduces a fair amount of redundancy in tickets, with several pairs of tickets that have the same starting point and endpoints only one segment away from each other. This means that a savvy player who knows the deck well enough can be almost guaranteed to draw tickets that he/she has already completed, essentially earning free points late in the game.
Europe introduces train stations, which make it much more difficult to shut a player out of a certain city. However, forcing a player to use a train station does reduce their bonus points at the end of the game, as well as jeopardizing their longest route bonus.
Luck-Based Mission: Any route that is significantly tunnel-heavy in Europe and especially Switzerland. Until you try to claim a tunnel segment, you don't know how many cards it will cost you. If it's a 3-train tunnel and you only have three cards of the designated color, you could get it on the first try, or waste several turns trying to complete the tunnel. Collecting one more card than necessary seems like the best way to mitigate this risk, but it's still a crapshoot.
Magnificent Bastard: Can be you if you block off the needed routes of another player. Especially bad in the Northeast Seaboard, where routes can be between two cities with one link between them. This can lead to a Precision F-Strike, depending on whether your route has been blocked completely or simply made more difficult.
While the train stations in Europe make blocking less of a viable means of bastardry, you can always wait for an opponent to build a longer link for you and then use a train station to avoid spending your own trains on it, theoretically allowing you to complete more tickets.
In Marklin, the passenger cards enable you to gobble up points by riding your opponents' train routes, stealing tokens from cities they thought were "safe".
In Switzerland, since discarded tickets don't go back into the deck, you can take a turn drawing tickets for no other reason than to waste routes that could be easy points for your opponents. If done right, this will stress out your opponents as they fret over whether it's more important to complete the missions they're working on, or take new tickets before the deck is completely empty.
Ask someone who is relatively new to the game to play India with you. Watch them build a nice, neat circle around most of the board. Block off the last segment of that circle. (You may want to brace yourself for the inevitable table flipping.)
No Export for You: Originally the case with Nordic Countries, which was initially released only in those countries. Popular demand led to a wider release.
The passenger tokens in Marklin are derided not so much for their effect on the gameplay, but for the annoyance of having to set them up each time, and the fear of losing even one small piece (unlike the trains, there are no replacement tokens).
That One City: Every board seems to have at least one city that's inconveniently located, such that it's out of the way of most other routes. Drawing a ticket that leads to such a city late in the game can be your downfall.
In USA, it's Miami. Getting there requires 4, 5, or 6 cards of a matching color, depending on whether you route your trains through Charleston, Atlanta, or New Orleans.
Europe has Edinburgh, which is only connected to one other city, London.
Geneva, and just about anything near the Italian border, can be a royal pain to get to in Switzerland.
Legendary Asia has Khabarovsk, in the northeastern corner of the board. You either need five matching red or black cards to get there from other mainland cities, or you have to take at least two ferries to get there via Seoul and Vladivostok. The ferries also make getting to the island nations (Japan, Taiwan, Sri Lanka, and for all practical purposes, South Korea) a bit of a pain.
Team Asia has Kathmandu, which is close to the middle of the board but only accessible via deceptively short tunnels that could cost you up to six extra train cards.
Africa has Addis Ababa and the three countries off the map at the northern end of the board, which can be difficult to reach from the more populated southern areas. Tchad in particular is at least two segments away from any named city. And Madagascar is (naturally) only reachable by ferry.