Someone has been quoted as describing a soldier's life as long periods of intnse boredom interrupted by brief episodes of stark terror. Most soldier's lives consisted of training and training and more training and then brief periods of intense combat, in both World Wars.
The major difference in the European theater was that in World War I there were often long stalemates with both sides building huge trench networks along with the use of poison gas which was not a factor in World War II.
In both wars most infantrymen used semi-automatic rifles with bolt action single shot rifles being more common in World War I. During World War II more automatic weapons were used with infantry often using submachine guns or smaller automatics such as the BAR.
Major differences between them and today have to do with weight, rate of fire, size of ammunition and complexity.