Although the trend was really just starting, the groundwork for "appliance" or commodity computers was being laid in the late 1970s. Apple, for instance, introduced their floppy drive which made Apple computers far more friendly and more usable then they were with just a tape interface. The disk drives speed and capacity (relative to cassette tape) also allowed the Apple II to vie for more of a share of the corporate computing arena.
Radio shack introduced their disk drive for the TRS-80 Model I as well as some revisions that made it more useful. The TRS-80 was successful for reasons beyond its features, however, since it was readily available from a store that most people were already familiar and comfortable with.
The S-100 bus standard became far more prevalent over the course of the year and dozens of companies joined the pioneers in producing S-100 machines and add-on cards. Memory prices also started trending downwards which allowed the basic computers more storage and, therefore, more power.
One of the most significant developments was, however, the birth of the software industry. This wasn't actually an event that occurred in 1978, but it was a trend that became more than obvious during this year. In 1975 you could count the number of independent software houses dedicated to personal computers on one hand (you usually bought your software from your computer manufacturer or, more likely, you wrote it yourself). For the next year or two that number didn't increase too rapidly. By 1978 there were dozens of companies producing all sorts of products from games to database applications.
WordStar, the first "real" word processing software was, for instance, released in 1978.
By the end of 1978 there were around 200,000 personal computers in circulation worldwide as compared to maybe 10,000 two years earlier.