Well, nothing really. But nothing's perfect. And nothing's ideally suited to every potential buyer. My 540 is a great car, but there were a couple of things I could do to make it a better match to my needs. You may find these ideas useful for your own car, though you may not be able to make exactly the same modifications. Before making any permanent alteration to your car, make sure you understand exactly what you're doing and know it's safe.
In the not-terribly-significant department, I installed a cargo net in the trunk and anti-theft wheel lugs on all four wheels. Both are factory parts. The cargo net is definitely useful for keeping small items of luggage, groceries, etc. from sliding around in the trunk, and it's easy to use.
The anti-theft wheel lugs come with their own drive adapter that allows a standard lug wrench to work with their unusual head pattern. It wouldn't be impossible for a thief to remove these lugs even without the adapterand I'm sure there must be thieves out there who carry these adapters around just in casebut they'll be a minor deterrent, at least.
A Mag-Lite mount (720x540 JPG)
I used short self-tapping sheet-metal screws in pre-drilled holes to install standard Mag-Lite mounts just inside the driver's-side door. Easily done, and not in the way of my feet. Note that the light is equipped with the optional hexagonal end cap. Though you can't see it, the cap is holding a red filter over the standard clear lens. The filter allows the light to be used at night without destroying the user's night vision. When the filter is not needed, the cap slides off along with the filter, producing the normal white light.
The Mag-Lite mounts are cheap plastic, and I broke a couple on my previous car. I've considered building some kind of sturdy custom mounting system, but I think it's easier to replace them occasionally. I've considered replacing the Mag-Lite with a better flashlight, too. There are some very impressive units designed for the public-safety market. Some of these professional models are much brighter, but they're also much more expensive.
A fire-extinguisher bracket (720x540 JPG)
A more significant modification is shown here, the installation of a small automobile fire extinguisher (a First Alert 5:B-C model from the local hardware store) on the front of the driver's seat. On the optional sport seats I have, there's an extendable thigh support that comes out far enough to provide a very natural space for an extinguisher.
Installing the bracket was a non-trivial task. There's a convenient cross-brace that's large enough to hold the bracket, but the mechanical bits of the seat are right behind the brace. I positioned the fire-extinguisher bracket where I wanted it, then marked and centerpunched mounting holes. Before drilling each hole, I slipped a small, thin (0.060") sheet of stainless steel up behind the cross-brace to protect the components of the seat.
I used a 13/64" drill to make the mounting holes (you may want to use a 3/32" or smaller bit to make a starter hole before drilling the final hole; this will help keep the bit from wandering). To mount the bracket, I used #10-24 flat-head countersunk stainless-steel bolts trimmed to 5/8" long. Behind the cross-brace, I used #10 washers and #10-24 nylon-insert locknuts to provide vibration resistance. These were a lot easier to use than lockwashers, but even so, installing them was very difficult. There's hardly any space behind the brace. I finally gaffer's-taped one washer/locknut set into a short open-end wrench, which let me get them started and snugged down in one operation.
Please note that these instructions may not be relevant to your car, even if it seems similar to mine. Always be sure that what you're doing is safe and correct before you do it.
The mounted fire extinguisher (720x540 JPG)
Here's the final result. It fits fine, it looks good, it's solid, and it's completely out of the way.
There are a few more things I'm considering.
For example, the center console on this car consists of a sliding armrest with two pads on either side of a shallow tray designed to hold a cellphone, sunglasses, or other small items. It's pretty much useless for storage, though it occupies a significant amount of space. I've heard about a couple of alternatives from BMW: one a sliding armrest with an internal storage compartment, the other adds a shallow storage compartment to the console with a hinged (but non-sliding) cover. There may also be other options. I'm still thinking about this one.
I'm also considering a few options for increasing performance. Dinan is preparing a new engine-control program for this car, and is working on a supercharger package. I think the supercharger is going a bit too far, though. Jim Conforti is also working on a new performance chip for the E39; keep an eye on his company's Web site, Bonneville Motor Werks, for more information. (I'm also planning to get a Valentine One radar detector. :-)