I would leave the feet as is and wrap the bottom with some base board moulding that relates to the crown and nail into the feet to look like the whole bottom is solid, otherwise you are looking at a really complicated hack. Or you could wrap the individual sections of feet with crown cut to make the current feet look like fancier feet, there will be hard angles at the corners.

To get a more substantial look, you could expand on your crown molding idea by repeating that treatment at the bottom of the bookcases. Only, this time, you would use a baseboard molding that complements the crown molding at the top. This will encase the feet, visually bring the piece down to the floor and giving it a more solid appearance.

i think a color change would be cool, but not into crown and/or baseboard because the piece itself is about simple lines.

http://prettypegs.se/

I bought some pieces from the Hemnes series and added molding to the bottom, top and in between. I ended up an entire wall with a built in look.

http://www.no29design.com/2012/04/built-ins-part-one.html

http://www.no29design.com/2012/04/all-finished-basement.html

@CHRISTINA FROM NO. 29 DESIGN, that is REALLY beautiful--great job!

Just my preference, but I would experiment with color before cluttering that classic shape with curves. Painting the back panels in a bright gloss (deep turquoise or cinnabar red) or covering them in a graphic wallpaper (Osbourne & Little style) would bring dimension. Changing out the glass panels for milky laminated glass might be a good tweak. Changing some of the shelves to glass and punching in downlights on the top of the units...

Wow, I like them a lot as is.

How about doing something with the backing of the shelves? (If it were me, I'd consider imitation gold, copper or silver leaf.) Depending on what you put behind the glass doors, you could frost them or cover them with fabric or something...

I would suggest painting or wall papering the backs of the shelves. You could have fun with color or print. I have even seen a bookshelf where each shelf back was a different color going down the rainbow or you could do tonal colors from light to dark at the bottom Here are some ideas:

http://pinterest.com/pin/100416266661525527/
http://pinterest.com/pin/100416266661764106/

Or just leaving the pieces as is but paint them a cool color.
Here is an ikea hack in a really nice bright color:

http://pinterest.com/pin/100416266661211481/

The built-ins Christina did were amazing though!! Love that idea!

An easy way to get some colour at the back of shelves is to wrap some fabric around cardboard, or wallpaper it. If you cut the pieces just right they should lightly friction fit without a major commitment. Easy to change or get rid of if you decide you don't like it. You could maybe attach some base molding with double sided tape instead of nailing but I don't know if the residue would be a problem to clean up. You can use fabric curtains inside the glass doors too. A friend has different sets of curtains for her glass cupboards, can change them quickly depending on mood or season.

I can see adding decorative molding around to obscure the blocky feet, on the front and both sides. A color change might be all you need. Other than the dark color and the fact that they were too wide for my inset wall (the only reason I went with Billys) I LIKE the Hemnes bookcases.

@CHRISTINA FROM NO. 29 DESIGN those built-ins look fantastic.

Covering the backs of the cabinets adds some visual interest as well. I added some fabric shelf paper to my Hemnes pieces.

http://www.no29design.com/2012/05/shelf-paper-isnt-just-for-shelves.html

This is really really lovely as is. A very nice piece. If i were going to make any change — I would only paint it light gray or a beautiful white or a chalk black or a black blue. I wouldn't hack for hack sake.

I agree with Tashr, although I think you could definitely get more adventurous with colour if you wanted to. But I don't think messing with the straight lines of the piece is a good idea.

another idea for the feet would be to find (or cut) some curvy little brackets. The right angle would fit in the angle between the foot and the top, and the curvy bit would add an interesting shape.

@Christina from No. 29 design, what a great job! I like the idea of hacking them with something along the back; I'd go for a muted metallic: subtle pewter or dull copper or gold, depending on your color scheme.

My husband and I did a hack on a mismatched Hemnes bookcase with glass doors. We had to match the shelves that were white to the black ones and then added color to the back by painting textured paintable wallpaper to the back. More info and pictures on my blog: http://jessicanicolehall.wordpress.com/2012/03/27/before-after-mismatched-hutch/

@Christina, you have taken over this thread- shame on you! :-D JK, but seriously, that built-in is ridiculous! It looked awesome!

You could add pattern to doors with glass window contact paper (designyourwall.com) and/or few curvy vases/sculptures/artwork on easels to introduce some feminine lines. BUT, if you insist on covering feet, you could opt for antique brass toe caps from houseofantiquehardware.com. They also have a bunch of cool hinges, if you're feeling really adventurously crafty, screw a few decorative hinges onto the two exposed sides of each leg and you have instant embellishment.

Mirrors and color are your new best friend. Mirror the backs on either side, all the way to the bottom. In the middle, you should paint the back either white, blue/grey, or kelly green for a nice pop behind the glass. I agree with others above that you should add molding to the base to make it look more substantial, the legs look weak, cover them. Add a simple molding to the top, not too curvy because the sides and the shelves arent thick enough for such an ornate molding, you wont be fooling anyone. Change the hardware to mirror handles, or polished chrome knobs. Decorate lightly, cover books in fabric/paper that corresponds with the color painted in the middle. If you can, hang a small pictures, on either side in the middle of the bookcase, that blocks the shelves a bit, its a great look and adds space for small art pieces because you lost so much wall with that unit. hope this helps!