Anne's point about skill is an important one. There are good and bad hand sewers. There are good and bad machine sewers. My wife sews a lot of her clothing, and when she has anything that needs hand stitching, she brings it to me, because my hand stitching is superior to hers (something about hours and hours spent sewing pleats...). However, she greatly outshines me on the machine. Ask me to sew anything other than a simple straight running stitch on the machine and I'll mess it up (and even then, it won't be the best running stitch you've ever seen).

But, assuming the maker of the hand-sewn kilt and the machine-sewn kilt are equally skilled, here are some average differences you are likely to notice:

1. The hand stitching in the pleats will be nearly invisble. This is perhaps the primary "trademark" of the hand stitched kilt that most kilt wearers will look for.
2. Machine sewn kilts tend to be top-stitched, meaning the stitching will be visible in the pleats. To a lot of kilt wearers, this is the sign of a lower-grade kilt, which is why most machine stitched kilts you see will be marketed as a "casual kilt."
3. The machine stitching is actually a stronger stitch. Believe me, I know. I've had to rip apart both hand stitched and machine stitched kilts for reconstruction or alteration, and it's a bear to take out machine stitching. (But reference what Steve posted about the interfacing in most quality hand sewn kilts).
4. Machine stitching is faster and more economical, therefore your machine stitched kilt will likely cost less for the consumer to purchase.
5. Because machine stitching costs less, you are more likely to see it used on kilts that have been designed to cost less -- in other words, there will more likely be other things about that kilt designed with cost-cutting in mind, such as absense of interfacing and lining; it may be sold off-the-peg in standard sizes rather than made-to-measure, etc.

Not all of the above is universally true, keep in mind. For example, the "casual kilts" we sell at the Scottish Tartans Museum are machine stitched, but they are made-to-measure just like our standard kilts, and have a lining, etc.

And the machine-stitched kilts you'll buy from USA Kilts are not top stitched, because Rocky and Kelly have learned a method to sew the pleats that involves stiching the pleats on the inside, making the seam nearly invisible, like on a hand sewn. I'll have to let Rocky go into more detail about that, if he likes.

At the end of the day, though, for a lot of people, the attraction to a hand sewn kilt is just the idea that an individual craftsmen spent hours upon hours of personal time and effort to create the garment for them. The same thing that attracts people to Harris Tweed, woven by hand, not on a mechanized loom. The same thing that attracts people to hand knit kilt hose, rather than machine knit. It's that human element.