Paint out the trim and panels.

Give em' a coat of high gloss black and change out the hardware. If you're really adventurous you could try to re-veneer them altogether.

All the above +
sand, wood fill the recessed portions for a flat door look.

Change out the hardware at least - who says you need a traditional round handle? Instead opt for a sleek handle (such as the METRIK or LANSA from Ikea). Then use a magnet to keep the door closed when needed. This would also apply if you replaced the doors for less builder basic ones. Good Luck!

You could make a thin luan panel that goes over the entire inner part of the door's recesses, and then put a very simple trim (square edged?) around that.

Or, you could sell these online so they get reused and then put the money towards the doors you prefer.

Think about zinc or copper cladding from rotometals... that could be cool. With a hand forged steel or leather pull?
http://www.artfactory.com/door-pulls-contemporary-style-hand-forged-hh199-p-2537.html
http://www.mockett.com/furniture-hardware/drawer-pulls-knobs-door-handles/leather/dp420.html
or if they are not solid wood, replace them with cheap luan doors painted with chalkboard paint with a cool door pull.
myself, I love old, old doors with a patina so I would look at salvage...
pam h
howtorunyourlife.blogspot.com

It seems there is only so much you can do with a door to change it up. I remember AT posted some cool door decals a few months back that made your door look like it was a stairwell, escalator, etc. We replaced 2 of our more traditional doors with wall slide glass doors. Instant face lift!

I would not fill the panels in with filler. That much filler will shrink and crack. It will also look very bad. Also if you add to much thickness to the doors they will not close properly with out modifying the jambs. If the doors are solid wood, then I would cut out the top panel and square off the top. Then I would install a piece of glass in the upper panel. You can then frost the glass with a vinyl film. I would do vinyl because it is not permanent and you can cut a pattern into easily.

"Or, you could sell these online so they get reused and then put the money towards the doors you prefer."

I really think this is the best solution.

a very modern design tip would be to paint the wall, door trim and door the same color for a minimalist monochrome effect. here is an example: http://www.flickr.com/photos/10177557@N08/3544071954/
For example, you could paint the wall, trim and door all the same red paint (or whatever color) and the trim/door would blend with the wall surface to read as one cohesive plane.

I have no idea how well it would work or how easy it would be...but depending on how much privacy you like, it might be possible to pop out the top panel and insert smoked glass to open up the space.

Have a rectangle piece of steel or aluminium cut that is just big enough to cover the dato'd detail but still leave a chunky border of plain door all the way around. Attach with screws and washers at each corner that are just big enough to show that you mean it. Or a piece of 1/8" luan like someone else said, painted in a contrasting color.

Doors are surprisingly cheap. Buy new doors.

Even if you go with the pricier options, the impact should be worth it, and not much more expensive than all of the sandpaper, filler, gloss paint and other stuff you'd have to buy to "make over" the existing doors.

Frosted glass with a wood frame might suit what little I can see of your decor. They will also look VERY swanky.

Upholster the doors with pleather. Trim them out with studs. Get some retro knobs.

I hope the words I write will make sense...I can see what I am trying to say in my head ;)

I think where the panels have been created- add a fairly wide wood trim to disguise the shape of the panel- particulary the top one that has a less than modern shape. So, what I mean is, add a flat square piece that is slightly larger than your panels, either like a picture frame or a completely flat panel like a square or rectangle. Then paint the panels to match your door colour- white is still nice- it is the shape of the upper panel that is dating your door. Also, change the knob, Home Depo has loads of styles to replace the round one you have now.

Just replace them; doors are fairly cheap, since this looks like a developer rehab they are probably totally standard, you can probably get them prehung or ready to hang (w/o frame) already. White & paintable or a cool veneer. You can reuse the silver knobs or go to a sleek lever.

Don't do magnets, you won't be happy and if you have to sell, will be a hard sell. I don't know if a salvage place would want those doors, but a reuse one would.

You can probably replace all 3 for under $100 with flat (no panels) doors, if they are typical hollow interior doors. Might not be worth the effort to fix/uplift the current ones.

Oh please, just paint them.

I highly recommend visiting you local Restore by Habit for Humanity. We buy a lot of doors there.
I don't think we've paid more than 25.00 for each. merely lifted the old door off the hinges & slid the new one on. It was so easy and not labor intense.