Pretty detailed topic. If you're after an all-rounder, you basically want one with an IPS panel and good refresh rates at a decent price. Generally these will have more accurate colour reproduction and wider viewing angles than cheaper TN panels, but usually sacrifice on refresh rate.
Beyond a certain level you start to branch out into either serious gaming or professional photo/video editing monitors. The latter can cost thousands apiece at the top end for an Eizo or NEC. Come back a peg to around $1K and ASUS (ProArt series) and Dell make very nice monitors often marketed as "professional", although these will be less than optimal for gaming. ASUS also make very good gaming monitors for around the same price.
Come down another level and you can get a very decent all-rounder for about $600. A rough guide to bang for buck is Samsung and BENQ, then LG or ViewSonic, then Philips, Lenovo, Acer. Be wary of brands outside the mainstream.
Things to consider
size - has to be big enough for your needs AND fit in the space allotted. 30+ inch monitors take up a lot of real estate...
connections - check your PC/graphics card outputs to see what you have (HDMI/DisplayPort/mini DisplayPort/DVI) and make sure the monitor has an input to match. And check what cable(s) are in the box with the monitor
resolution - check the specs of your PC graphics card to see what combination of resolution and refresh rate the card can handle. Probably not worth considering any resolutions under 60Hz due to flicker. The higher the resolution, the smaller the text will be, and most monitors only function effectively at one resolution. Make sure you can live with the scaling.
aesthetics - the value people place on this varies, but some of the designs are pretty basic and ugly/clunky. Thin bezels (frame around the screen) help.
specs - dot pitch, viewing angle, brightness, contrast ratio, power consumption, number of colours/% of colour gamut are other specs worth investigating (brightness & contrast ratio can be meaningless, a bit like power ratings for hi-fi speakers...)
Lastly I'd never buy PC equipment from a Harvey Norman/Officeworks/JB Hi Fi due to markup (although I might browse to see what the item looks like). The best prices you'll get are at somewhere like CPL (great range & prices), MSY (great prices but usually only stock cheaper models), Scorptec (great service, OK range, not so cheap) or PC Case Gear (happy medium of price/range/service).
Generally price is an accurate indicator of quality, but be careful of paying for features you don't need. Positive reviews from two or more sources are a good sign.
If you really can't be bothered doing research and you're looking to spend $600 or less, the best Samsung within your price range is usually a safe purchase.
