I guess it depends on what you want to use the monitor for.
-Resolution, contrast and response time is ussually the 3 main features I look at.
-Also the feature of the monitor is important...i.e. what devices do I plan to plug to it.
-Also, having a good viewing angle and if it has a good anti-glare filter or not is important.
-Also make sure its ENERGY STAR qualified.
-Also the warranty that comes with it is a telltale of how reliable the monitor is.
-Sound might be important also if you dont have pc speakers.
I like to play video games, edit photos and watch hd movies on my monitor so I need one with fast response time and high resolution.
The Samsung 24inch P2570 is a pretty decent monitor and at a decent price for around $300:
-70,000:1 dynamic contrast.
-170° horizontal and 160° vertical viewing angles.
-Ultrafast 2 ms response time.
-1920 x 1080 maximum resolution.
-Connector: DVI-I, HDMI, audio out, optical out. Analog RGB / DVI
Of course, if you don't plan to do much with your monitor, you can get one for probably much less money.