There are several factors that are of prime importance.
1. How long will the battery last when I charge it? 650mah or higher is good. The higher, the better. Most mods fit this description easily.
2. What kind of atomizers and cartomizers can I use and what style thread should I buy? The 510 thread is highly popular. You can use just about any carto or atty with it.
3. What do I have to spend to get a good vaping experience? You can achieve that with an under $35 Riva Kit, a $50 eGo kit, or any number of inexpensive ecig kits with larger batteries. In a mod, be prepared to spend $75 - $200. That said, you can also buy a variable voltage box mod from Madvapes for $35. You will need a charger and two 14500 batteries for that, which will run you about $30 additional. The MV box will be plastic, but will give you virtually the same vaping experience from a taste and vapor standpoint as an expensive mod. You can vary it from 0v - 7.4v and use all the 306 and 510 cartos/attys. There are adapters that will let you use 801, 901, 808, and most any atty thread.
Past that, looks and design of the housing enter into the equation. Do you want something elegantly designed or something that looks like a flashlight or metal box with a carto sticking out of the end. Do you want a juice feeder or something that uses cartos that you fill separately?
and I've heard good things about the vmod. I'd like to have one -jealous- 
