I include these as a spoiler so folks who don't want to know can avoid them.

- Captain_Crunch mentioned a good one. Barovia is actually a part of Strahd's mind or a reflection of it. As an example, he destroyed the village of Berez in his anger over the events there by simply willing the water in the river to rise and inundate the village. However, he is not really consciously aware of this, he can't just change any detail on a whim, it is more of a subconscious effect.

- Strahd made a deal with the entities in the amber temple - the amber temple contains a library with every spell from the PHB. Strahd could easily be a higher level spell caster than he is but has simply had no challenge or motivation to improve. You can use this to modify Strahd's stat block for the end scenes if required. There are enough milestones in the adventure for the characters to reach level 12 or 13. In addition, the magic items available are exceptionally effective against undead and vampires. A high level party with the magic items will defeat Strahd easily in only 1 or 2 rounds of combat so depending on how you run it and how much of a completionist your players might be, you may want to modify Strahd's stat block by the end.

- The castle is a bad place to go. There are gargoyles and the entry room has 4 red dragon wyrmlings that attempt to prevent characters from ever leaving the castle - though the doors open invitingly if the characters approach. A lower level party will likely TPK if they enter the castle and try to leave if they take the coach they find at the crossroads. It is important to emphasize to the players that Barovia is dangerous and contains creatures and encounters that they will not survive if they are too low a level. Ideally also mention that the information they need to survive is likely available in the world but that the DM will not be saving the party if they choose to follow a dangerous path. On the other hand, you could choose to make places easier if the characters happen to choose to go there but ideally that isn't how CoS is structured.

- Take a look at the Tarokka deck reading and stack the results. For example, there are a number of possible companions and their utility varies A LOT. Some would be very useful and others it is hard to see how they would be any use in a confrontation against Strahd unless you give them additional abilities or roleplay options. Similarly, some of the items could be randomly placed in the castle which could also be problematic depending on whether they find the item before encountering Strahd or not.

- Related to this, there are a couple of items that are IN the castle - the skull of the silver dragon and the holy symbol that will release the ring of regeneration in the tombstone in the graveyard at the Abbey in Krezk. These are actually quite cool in terms of effects and the ring is useful but won't break anything. However, the odds of the characters finding the spot where these are needed, going into the castle and searching it, finding the items and then leaving the castle to complete these side quests before going back to deal with Strahd are almost nil. I ended up giving the holy symbol to the priest in Vallaki and he gives it to the party as reward for recovering the bones that protect the church. The priest is related to the original owner of the holy symbol/key so it makes narrative sense. As far as the skull goes, there was a raid on the castle in recent years lead by a mage from the prime material. The rebellion was crushed, Mordenkainen the mage lost the battle with Strahd and is currently running through the hills to the north hiding and suffering from madness. However, a large group of Barovians did invade the castle so I had them take the skull and had the Ravens hide it in the basement of the Wizard of Wines winery since they are probably aware of the origin and possibly aware of its usefulness. Again it made narrative sense and made the item available to the party when they go to Argynvostolt.

- Consider if you plan to add any magic items. CoS has a very limited collection of magic items and most of those are found in the castle in a couple of areas. I was running it as written and one of the players who was running an archer was getting very bothered because they could not find a magical bow (there are quite a few enemies resistant to non-magical damage). I ended up putting in a +1 Longbow that the character was able to trade for from the Vistani ... however, usually, only items in the PHB worth 25gp or less are available in Barovia and they sell at 10x the going rate. The only source of outside goods is the Vistani and they are very scarce and very expensive. If run as written, some characters will still be wearing some of their starting gear by the time the module ends. Players with characters that "require" a specific magic item to complete their "build" are likely to be disappointed unless you specifically add it for them.

- Consider Strahd's motivations. He is bored. He wants to give up ruling Barovia but can't find a suitable successor. This is impossible but he doesn't know that. Strahd can easily kill new visitors to his realm if he wishes but he doesn't because they are both his play things and if they prove worthy, they never do, possibly his way out. So, Strahd will constantly test the newcomers, challenge them with dangers, he doesn't care if they live or die because there will always be more but there is no point in killing them out of hand. It is this overconfidence that will be his downfall. However, if the party does develop into a real threat, Strahd is too smart not to take actions. He is aware of the items the party finds and may decide to control the circumstances of how his encounter with the party goes. In my game, Strahd recovered his plate armor that he had animated and set to defend the castle, he recovered his +2 great sword that he had left at Argynvoltholt, and he studied at the Amber temple, increasing his magical ability, learning new spells and changing those he prepared. He also went as far as creating an additional vampire and vampire mage, and recruited the assistance of some of the druids from the hill. This set the scene for a final battle where he had the druids cast silence and fog cloud preventing most of the items that required sight or emitted sunlight from affecting him and his allies. The silence was sufficient to make it difficult for the party to cast spells until they found areas that weren't silenced. He also cast tensor's transformation on himself (one of the new spells he had learned). This was all required since the party was about 12th level by the time of the final confrontation.

- The hags at the bonegrinder mill can be challenging. I've heard of TPKs but my group didn't have any real trouble. (These hags make dream cookies but one of the ingredients is the ground up bones of children sold by their parents in exchange for the temporary escape offered by the cookies).

- Most of the Barovians do not have souls. They are essentially figments of Strahd's mind. Souls are prevented from escaping Barovia and will eventually be reborn so some fraction of Barovians do have souls.

- Finally, there are a lot of possibly disturbing or triggering horror themes in the story. Strahd is a vampire infatuated with one soul which is trapped in Barovia and he stalks her everytime she is reborn. There are horror fairytale elements like eating children. The werewolves forcing children to fight to the death. The insanity and asylum themes of the mongrelfolk of Krezk. It is a bit more than just the usual undead and you may want to be aware of it in case it might bother some of the players.