Buying boarding boots is about the same as buying ski boots. You should try on every set of boots in the shop, then go to the next shop, and the next. Once you know what fits and what does not - then look at your short list again.
For the newbie, there are two distinct types of boots - soft and hard (plastic). If you don't know this, you don't want plastic boots... as plastic boots go on a race board.
Most boots are fairly durable. Like anything there are a lot of features you can look for. Boots can have two sets of laces (inner and outer) or a pull string on the inner. The toe and the heel of the boot can be tapered upward to prevent toe or heel drag. The amount of support can vary from boot to boot. Park oriented boots are normally a bit shorter around the ankles than free ride boots. Have a look at how tight you can get the boots around the your ankle.
The thing to remember is - when you ride well - you ride though your feet as if you are standing on the board. So, like a normal pair of shoes, you should be looking for something which is comfortable to stand in as your first priority.
What can aid in this greatly are a pair of footbeds, such as Superfeet (awesome, $30 to $40 typical).
The last thing to look as is, are these boots going to fit into your bindings properly.