Other things to consider:
Stall type- slant loads are usually considered a little easier for the horse to ride in than straight loads, though the stalls can be a little bit cramped in length in some models. (I only read a little of this page but it looks like an interesting read on slant load stall sizes: Slant Load
) For straight loads, adequate stall width is usually not a problem, but you should still check stall length. I avoid manger style straight loads completely- I've heard of too many instances of horses panicking/becoming unbalanced during a quick stop, rearing up into the manger area, and getting stuck.
Fully enclosed vs open- Fully enclosed trailers are very popular and often viewed as "better" but open (stock type) trailers give maximal air flow, which is important even on windy, rainy, cold days. I started out looking for a fully enclosed trailer, but ended up buying a stock type and couldn't be happier with it.
New or used- Figure out what your budget is and look at what you can get for it in both the new and used market. I got very frustrated looking at used trailers that hadn't been maintained well- 10+ year old trailers still with the original tires, never been serviced, soft spots in the wooden floor, etc. I ended up getting a new trailer that I knew was in great shape (with a warranty in case anything did come up!) A little less fancy than I had originally hoped for, but with a safe and generously sized horse area.
And finally... is the tack room well sealed? I paid special attention to the sealing between the horse area and the tack room to make sure hay/dust/etc wasn't going to seep into the tack room. Unfortunately, I didn't pay attention to the tack room door itself, which has ~1/2" gap on all sides of the door :headdesk: This is the one thing I dislike about my trailer.