Transitioning from low or mid stakes poker to high stakes can be quite the jump for some players who aren't ready for the inherent differences in how the move to a much larger sum of money going into the pot can influence play style. The behaviours of the other players at the table and the changes that the increased risk that each hand holds can alter the course and pacing of the game and require different approaches that may not have been taken in lower cost games.
Depending on just how wealthy the high rollers are sitting at the tables, chip stacks may be considerably smaller, a fact that belies the dramatic increase in value that they hold. For more inexperienced players, this may cause increased stress as the visual cue tells them that they are starting the game or, just a few hands in, running low on funds, causing them to play far more defensively. Players who are comfortable playing at this level are able to overcome this and inherently know the dollar value that the colours are worth, letting them focus on their actions with just a glance at player stacks and pot.
As for those sitting at the table, moving up the ranks means that players who have both natural talent and learned skills will be more common, with weaker players rarely making the jump to tables and games that have higher risk to their bank rolls. This means that you can't count on people overplaying their hands with a false sense of bravado. Instead, opponents are more like to play conservatively unless they are absolutely sure they are going to take the hand, push another player into overplaying themselves, or be able to force someone out of their seat.
For those that take their poker games online, players of all levels are more likely to be using tracking software than not. The chances of being up against people who are using additional programs to keep tabs on all sorts of statistics and analyze player habits are even higher when there's more money at stake. Familiarizing oneself with these by testing and trying them ahead of time at lower stakes can lead to much better results, especially because there are free options to try them out. Similarly, learning to multi-table using software can help towards being able to compete on the same level as those who are already well versed in high stakes.