As you must be thinking, Simulation problems are simply what this category's name means. You are asked to solve a problem following some rules. Generally all the rules are covered in the problem description.
So, following the rules given is the main point when solving this kind of problem, the second most important action is try to realize possible exceptions that may occur, if you don't do it, this mistake will centainly take you to an Wrong Answer!
There are some points to consider when talking about difficulty of Simulation Problems
1. Simulations problems get harder as the number of rules increases. This causes the code to grow up and get more illegible (lots of conditions, breaks, iterations, etc., all in a mix).
2. If the simulation is always going forward, step by step, it's much easier to get solved than one that needs to step back.
3. Simulation problems that contains two or more activities running side by side, where the results of each activity have influence over the others are difficult to be simulated, in my opinion, this is the worse kind of Simulation problem. Most of the cases will envolve points number 1 and 2 mentioned above, and will required some knowledge of another algorithm category, like Backtracking or Dynamic Programming.
This category contains problems that need real attention, some are very easy and some may cause you a headache cause of details and so on. Pay attention on your choice.