Calls that can be completely objective without player interaction would be easier to handle electronically. These would be obvious calls such as balls and strikes, fair/foul balls, and HR/not a HR calls over the wall. Calls that are somewhat subjective or require player interaction are still much more easily managed by a human umpire. For example, safe/out calls, catch/not a catch, fan interference, fair/foul off a player. There's still plenty of room for human interaction without being so dependent on human error for critical issues that would just as easily be handled electronically.