The following section is organized by the variety of photographic scenarios LANDSCAPE, MACRO, NIGHT, PORTRAIT and SPORTS photography.

Nature Photography

The Nikon D5000 provides pretty much everything you need for outdoor photography, including a huge number of "landscape"/"nature"-related lenses through all price ranges, like the good but expensive Nikkor AF-S 12-24mm f/4G IF-ED DX, the Nikkor AF-S 10-24mm f/3.5-4.5 or one of the best DX standard zoom lenses in Nikon mount, the Nikkor AF-S 16-85mm f/3-5-5.6G ED VR DX. In addition, the Nikon D5000 has a good battery life of around 400 shots on a single load which is sufficient for trips lasting several days.
However, there are some minor drawbacks like the lack of a DOF-Button for evaluating the depth-of-field before the exposure as well as the small viewfinder which is, admittedly, still one of the best in its class. Furthermore, with a weight of almost 590g including battery and SD card, the Nikon D5000 is no lightweight equipment and not suitable for all backpackers trying to minimize the weight and bulk of gear carried. Nevertheless, the Nikon D5000 uses the same Nikon's dust reduction system like the Nikon D90, which operates flawlessly and assures clean images without dust on your pictures.
Although the quite heavy Nikon D5000 has a lack of a body sealing and no DOF-button it is suitable for fairly difficult outdoor missions.

Macro Photography

More and more manufactures offer DSLRs with LiveView mode but only a few have a heart to combine this useful feature with an even more useful tilt and swivel display. Nikon follows this trend and present their first DSLR with a downward-opening display, which according to Nikon "makes it easy to capture images from any angle and still keep a steady grip on the camera." Furthermore, the vari-angle display can be swiveled and closed for protection of the LCD panel. In addition it is possible to magnify up to 6.7x in LiveView mode for more precise manual focusing, for example with great lenses like the superb Tamron AF 90mm f/2.8 Di SP macro without any significant flaws or one of the few third-party lenses that can play in the league up there with the big boyz, the Sigma AF 150mm f/2.8 APO EX HSM macro DG D
However, the Nikon LiveView AF works accurate but the speed of the contrast AF is one of the slowest seen so far and no real alternative for macro photography, especially for fast moving objects.
All at once, the Nikon D5000 is capable to produces great macro results in combination with one of the great macro lenses and available ring-flash units.

Night Photography

The Nikon D5000 doesn't offer something special for night photography compared to other competitors. The exposure button has to remain pressed during the whole exposure and If you require more than 30 seconds of exposure time you have to use the wired remote control MC-DC2 or wireless remote control ML-L3, which offer release adjustment for bulb exposure.
Nevertheless, the Nikon D5000 provides continuous shooting up to 999 frames, with a timeout between 1 second and 24 hours in self-timer mode which is an interesting idea for interval photography, e.g. for sunsets. The implementation of this feature is quite perfect compared to the Canon EOS 500D,where you can only define up to ten frames without timeout.
In addition, the Nikon D5000 has a TTL controlled flash socket for the use with the Nikon Speedlight series or Metz flashes for instance.
Regarding the "film speed" the Nikon D5000 is capable to produce very good results up to ISO 800 while ISO 1600 is still a borderline. However, in extreme low-light situations the Nikon D5000 can take advantage of an AF assist illuminator to assist the focus operation, but this is rarely the case as the AF speed convinces also with bad lighting conditions.
Unfortunately, Nikon doesn’t follow the trend of camera built-in image-stabilizers. Similar to Canon and Panasonic they provide lenses with their own built-in image-stabilizer, the Nikon’s Vibration Reduction (VR). This works fine but not really better than a built-in image-stabilizer.

Portrait Photography

There're lots of "portrait" lenses in the Nikon lineup. However, the primes, such as the 85mm f/1.8 or 85mm f/1.4, are all based on screw-driven AF which is not supported by the D5000. This is a bit of a limiting factor. Alternatively you could use the AF-S 50mm f/1.4, AF-S 60mm f/2.8 micro or the 70-200mm f/2.8. If we're just looking at the camera's capabilities the Nikon D5000 provides some portrait photography supporting features, like a face recognition system that can be switched on in live view mode. This is a nice gimmick but not really needed for more serious portrait photography, especially when thinking of the very slow contrast AF of the Nikon D5000.
As already mentioned, the viewfinder is quite small and the missing depth-of-field preview might be a reason to search for a better alternative when portrait photography is your main application.
However, the max X-sync speed of 1/200sec is generally fine for the typical focal lengths between ~50mm and 135mm used for portrait photography and with external flashunits high-speed sync. is also available. In other words - yes, in combination with one of the excellent portrait lenses and with lowering one's sight, the Nikon D5000 is suitable for portrait photography.

Sports Photography

With continuously up to 4 pictures per second, the Nikon D5000 is actually one of the fastest in its class and therefore geared for fast sport action shots. The memory buffer handles up to XX frames in RAW and even up to XXX frames in JPEG mode with a maximum shutter speed of 1/4000s, respectively with a X-Sync speed of 1/200s.
The AF speed and the accuracy of the Nikon D5000 are great, even in dark conditions and guarantee sharp sports picture even in continuous AF mode, although not at the level of the Nikon D90.
A wide range of Nikkor and third-party lenses are suitable for action photography - e.g. the Sigma AF 100-300mm f/4 EX HSM DG, one of the most impressive Sigma lenses tested to date. It is able to deliver a near-flawless performance with great resolution figures, low vignetting, low distortions, low CAs and a very decent bokeh.