- Screen Resolution - Having a good screen resolution is important for a nice looking experience. A low resolution screen is one of the easiest cell phone deficiency to notice. You need a resolution of at least 800 x 480 (WVGA) to have a good clear smartphone experience. High end smartphones with higher resolutions like 960 x 540 (qHD) are just that much more beautiful. Think of WVGA as the 720p of the phone space and qHD as the 1080p or the smartphone world. Anything lower and you are just "standard definition." And nobody wants that.
- Screen Size - There have been a lot of low end phones coming out with screens in the 3.2 inch range. 3.2 Inches is too small to take advantage of a smartphone's capabilities! Even 3.5 inches or 3.7 inches is pushing it in terms of usability when it comes to web browsing and general text reading. These days you want a minimum screen size of at least 4 inches. And if having a small phone isn't important to you a 4.3 inch or larger is an excellent choice.
- Battery Life - This is a difficult one to define because your battery mileage will vary depending on how you use your phone. You can find info for battery life while using cellular data, wifi data, and just talking. Smartphone have notoriously bad battery life, but some are much worse than others. Look for a minimum of 4 hours on cellular data, 6 hours on wifi, and 6 hours of talk time. Even with these numbers you may find your phone needing a charge by the end of the day, but you should be able to manage. The best phones get 5+ hours on cellular data, 8+ hours on wifi, and 8+ hours of talk time. A good place to fine battery life numbers are the charts you can find inside any smartphone review on Anandtech.
- Processor Speed - This spec is pretty simple. You need at least a 1 Ghz processor. Less than that and things will get sluggish on your phone. If you are looking for something with a little more zip you can find phones with 1.2 Ghz dual core processors.
- Data Speed - Only two things you need to know here: 3G and 4G. 3G speed is good but 4G is much better. While the difference between 2G and 3G was big, like the difference between dial up and DSL, the difference between 3G and 4G is harder to gauge, given that there is so much variance in speed depending on location and network. Depending on what service you have 4G will be 2 to 5 times faster than 3G.
For a more in depth discussion of cell phone specs, check out my Smartphone 101 post.