Kind of a combination of things....1)leans towards modern praise and worship style churches, 2)doesn't want to celebrate Christmas on a different date, 3) doesn't want to stand through the duration of worship, 4) had a coworker who was an Orthodox convert that was (in her words) rather legalistic in his observances.
Many parishes celebrate Christmas according to the new calendar - mine does also.
As well, while standing is our custom and tradition, many parishes also have (gasp!) pews, and the standing gets easier with practice. As long as you stand when appropriate, such as for the Gospel, no one should have a problem with anyone sitting down. (They shouldn't be paying attention anyway!
The Orthodox coworker may have "needed" to be rather "legalistic" in his observance - we can't really know the state of another person's heart or soul - as Blessed Theophan observed, we should "see to ourselves and our own sins." That said, "legalism" is often in the eye of the beholder. It may seem legalistic to a non-Orthodox person for me to fast on Wednesdays and Fridays, when it's really a necessary discipline that helps keep me focused and aware.
(As a reformed "praise" singer myself, I have to say that one of the things that set me on my search was the "mile-wide and inch-deep" praise songs. There seemed to be no place for sadness or struggle in them.)