Nature Photography

The E-620 has a very strong AA-filter which helps to control (although not to eliminiate) aliasing effects but it reduces the pixel level sharpness in RAW files whereas JPEGs are good straight out of the box. The sheer resolution potential is lower compared to other DSLRs. The dynamic range is also somewhat more shallow specifically in highlights but it's not really a significant issue.
Some may argue that the Olympus color reproduction is very vivid and "punchy" but, at the end of the day, this is nothing else but a color profile characteristic which can be set/mimiced/created for other DSLRs. In contrast to the camera there're a number of very high quality lenses provided by both Olympus and Panasonic which are ideal for high quality nature photography.
The AF performance is generally very good with a quite pronounced fine spot characteristic. This is a plus when it comes to precision while it's not all that great with respect to focusing on coarse, low-contrast objects. If needed the camera does also offer AF fine adjustments so the fine-tuning of lenses is fully in your control.
The well-proven Supersonic Wave Filter assures that the E-620 is well shielded against sensor dust - a frequent problem in outdoor conditions. Based on what we've experienced so far it is superior to other systems.
Due to the lack of a body sealing the E-620 is less suitable for extreme outdoor conditions than e.g. an Olympus E-3. However, most of today's DSLRs work reliably even in difficult conditions. The battery life is about average.

Macro Photography

The articulating display (100% coverage) is very convenient for macro photography. It allows easy scene compositions even in difficult setups (e.g. low level). Additionally the possibility to zoom in during Live-View mode is a blessing for macro photography - many users prefer manual focusing and a magnified view is essential for really accurate work here. The new contrast AF works very well although not overly speedy (nor is this really necessary for macro photography). Remember that Live-View reduces battery life dramatically due to the processing requirements by the camera.
The four-thirds system offers 3 dedicated macro lenses at the moment (Olympus 35mm f/3.5 & 50mm f/2 and Sigma 150mm f/2.8). This is not an overwhelming amount of lenses to choose from but it's certainly sufficient. Olympus is offering lots of different system flash units so this aspect is a non-issue.

Night Photography

Just like most of the competitors the E-620 features bulb exposure but unfortunately the exposure button has to remain pressed during the whole exposure. If you require more than 30 seconds of exposure time the wired remote RM-UC1 offers release adjustment for bulb exposure. The E-620 has a TTL controlled flash socket for the use with Panasonic, Olympus and Metz flashes.
For night photography you should choose a low sharpening setting - otherwise sensor noise will be quite apparent from about ISO 400 onwards. The quite efficient sensor-based image stabilizer increases the hand-held potential quite a bit if you want to avoid tripod-based photography.
The AF performance is generally pretty good but it can be advisable to use the strobe assist light in this situation.

Portrait Photography

The E-620 provides all features required for portrait photography. For newbies or casual photography it ever offers a face detection system and art filters - nice gimmicks but not really needed for more serious portrait photography. The flash synchronization is fast enough (1/180sec) for the typical focal lengths used for this purposes and Olympus offers several system flash units to overcome the rather weak internal one.
The selection of special portrait lenses is unfortunately very modest in Four-Thirds lands. To date the system offers only one high speed prime lens in the typical focal length range for portraits - the Sigma AF 50mm f/1.4 EX. This is equivalent to "100mm f/2.4" (depth-of-field-wise) in 35mm format terms which is "good enough" but not great e.g. compared to a 85mm f/1.4 on full format DSLRs. Olympus argues that the 35-100mm f/2 is suitable for portrait photography but even this large aperture lens it is only comparable to a "70-200mm f/3.5" in terms of depth-of-.field. To be fair - this is better than anything available for APS-C DSLRs (where a 70-200mm f/2.8 degenerates to a "105-300mm f/4") ... but at a price.

Sports Photography

The E-620 can shoot continuously with about 2.5 fps (4 fps is only possible in manual focusing mode). The number of continuous RAW images is limited to 5 shots whereas the number of JPEGs not only restricted by the size (and speed) of your memory card. The maximum shutter speed is 1/4000s is nothing out of the ordinary but it's usually sufficient for most kind of action photography. The new AF sensor system of the E-620 is certainly an improvement over the old 3-point AF system and suitable for basic sports photography. This is about average for a consumer grade DSLR.

The 4/3 system offers a number of lenses which are suitable for action photography. The Olympus 50-200mm f/2.8-3.5 SWD is surely one of the key lenses for serious amateurs here. There're also professional grade lenses such as the highly desirable Olympus 150mm f/2, 300mm f/2.8 or the 90-250mm f/2.8.