Excellent question, and I had to think about it for a while. For some quilt artists, the quilting is the key element in the art, or at least is equal in importance to the piecing. One such artist who comes to mind is Paula Kovarik, who draws elaborate designs with her quilting line. In her day job she's a graphic designer who has been drawing with writing implements for most of her life, which is obvious from the masterful imagery she puts onto her quilts. I suspect that artists like this think of their quilting pattern at the earliest stages of design, maybe even before they settle on a final composition.
By contrast, there are other quilt artists who regard the quilting as a necessary but mainly functional part of the process. I am one of them.
For me, quilting is sometimes fun, sometimes rewarding, occasionally an opportunity to realize my artistic vision, but often it's just plain work. I make quilts because I love piecing, and then I do quilting because the layers have to be sewed together. I usually wait till the top is finished before I start to think about how I will quilt it.
In addition, there's the logistical issue. Quilting a small piece is quite a different story than quilting a huge one. I do all my quilting on my plain home sewing machine, not even one with a large harp. Since my favorite size is somewhere larger than six feet square, it takes a lot of wrestling and manhandling to get anything quilted.
In the past I have done elaborate free-motion quilting on very large pieces, but the last time I did so, I found it so physically wearing that I swore I would never do it again. I love the effect on the quilt, but not the effect on me. It's easier to put a walking foot on the machine, roll the quilt into a manageable package and do a simple grid. That's my default method, especially since the straight lines complement the straight-line piecing of most of my work.
This piece had relatively large spaces that seemed to cry out for fancy designs. I realized that I would be bored silly quilting the same designs on the whole piece, so I decided to use a different design in each little segment (segments ranged in size from a half-inch square to about six inches square). Probably the best quilting I've ever done, but I don't think I'll do it again.
This is my default quilting pattern, largely for physical reasons, but also because the simple grid echoes the lines of my piecing. I like to use several colors in the quilting and allow the grid to become irregular. The lines are spaced anywhere between an eighth- and three-eighths-inch apart.
On tiny quilts sometimes I quilt very dense parallel lines, less than an eighth-inch apart. I love the effect, but if I used this on a huge quilt it would never be finished.
On smaller quilts I can be more adventuresome because the piece doesn't have to be rolled up and can be rotated more easily under the needle. Here's one where the piecing lines go on diagonals, so I quilted lines parallel to the piecing. I love the final effect, but it was quite an intellectual challenge to plot the lines to cover all areas of the quilt at the density I wanted.
On very small quilts it's easy to make beautiful curves with a walking foot, but you have to be able to turn the quilt freely while you stitch all the way across in one pass. If you stop in midstream, the line will have an obvious, awkward break in the curve and look awful. I wish I could do this on a larger quilt!
Bottom line, I wish I were physically able to do more ambitious quilting on the majority of my pieces, but they're just too big. I'm fortunate that the straight-line quilting that I can accomplish with relative ease looks pretty good with the kind of quilts that I usually make. But in the end, I don't worry much -- I think my quilts are judged on the piecing rather than on the quilting.