Legitimate basis for excuse by judge:

Health condition affecting ability to serve
English comprehension
Severe hardship that is unique to the juror, however remedy may be deferred service, not excusal. Likewise, travel plans are also likely to get only a deferred service.

The next step is that you could get excused by the judge, for cause:

Extreme personal bias
Inability to abide by rules and instructions

Or you could get excused through attorney preemptory challenges