Two Entrances are better than one! - Solo tents generally have just one entrance but 2 and 3 person tents can have two. The lightest 2 and 3 person tents make do with one entrance but that often makes it difficult for one person to get out of the tent without climbing over someone else.
With two entrances in a two person tent this problem is eliminated - also two entrances usually means two porches - so gear storage is much simpler.
With a 3 person tent the person sleeping in the middle will always have to climb over someone else - but the issues are much more tolerable than if there was only one entrance for all three people.
Extended porches - These are useful extensions to a tent that can be used to store gear and become very useful in bad weather when you need to shelter in your tent all day - the extra space is also a useful area where you can take off wet gear without taking it into the inner tent.
Porches are useful if you camp with a dog - he can sleep in the porch without clawing the groundsheet of the inner tent.
Tents with this extra space often have XT after their name. The extension is normally fixed - but some tents have detachable extensions, a few are even big enough to store bikes if you want them out of sight. Often you can buy small groundsheets that match the floorplan of the extension.
Extended Headheight - All tents quote a figure for the internal head height of the inner tent - this gives an indication of how spacious the tent will feel. You should also look closely at the design though - the maximum internal height may not be spread over a wide area - many of the lightest tents compromise in this area - so the height may be 95cm but the walls of the tent fall away sharply, and this head height ends up being theoretical only.
Other slightly heavier tents often have short cross poles that extend the area of maximum internal height and are much more spacious than tents without a similar structural pole.
'Fastfly' Setup - Some tents allow multiple pitching options, which include pitching with a footprint and flysheet only - this type of setup has many names but consists of the tent being pitched as a single skin tent with the footprint being used as a groundsheet.
This gives a very lightweight option when you want it, it doesn't protect you against insects which can freely enter the tent area, and it can sometimes be tricky staying on a groundsheet all night. But if you know conditions will be suitable for this type of camping you have a very lightweight shelter option.
Additional Pegging Points - Nearly all tents will have additional pegging loops which you can attach guylines to if the weather is going to be windy.
Manufacturers tend to supply the minimum guylines and pegs so you will have to purchase these as optional extras, however we think its a very worthwhile investment.
Pockets, Lofts & Storage - Most tents will have at least one internal pocket for storing small items such as glasses, headtorch, iPod etc.. In all my experience camping I wouldn't like to be without at least one pocket - if only to store the things that may get broken if I was to roll onto them in my sleep.
A gear loft is an optional extra with most tents but its a fantastically useful addition - fastened on the roof of the tent it can hold small items that you want to hand. Some also act as a light diffuser so you can use your headtorch as a tent light.