Some thoughts, a bit Canon-oriented because that is the system I am using, but this should not make much of a difference:

• Focal length is the main difference between macros. Given that you want to shoot at 1:1 (otherwise you probably don't need a true macro in the first place!), focal length dictates the distance from the camera to the subject; a longer focal length gives a longer working distance.
• When shooting macro, depth of field rapidly approaches zero no matter what.
• The max aperture of the lens is really only of academic interest for macro use; the rules of the game change in the macro world so that a nominally f/2.8 lens ends up with a max aperture of something like f/5.6-f/8 at 1:1 magnification. Just a nice thing to know :)
• Indoor or outdoor don't really matter. Exception: A weather-sealed lens will be a safer bet if it is outdoors and it is raining. Given that you have a weather sealed camera body, if not, ignore this point.
• Build quality can be better in more expensive lenses. Cheaper ones are probably more than good enough, though.
• A lens with a USM focus motor (or the non-Canon equivalent) are handier for general photography than those with the older "electric razor"-type focus motors or screwdrive focus. For macro you will probably use manual focus anyway so this is not particularly relevant.
• Some macros have internal focusing, like the Canon 100mm f/2.8 macro. Others, like the Sigma 105mm macro, grow longer (a lot longer) as you focus closer. This can make a difference to the ease of use.
• Some macros like the Canon 100mm f/2.8L or the Nikon 105mm f/2.8 VR have internal stabilization, most macros do not. This is of some slight (very limited) help in a macro photography setting but can make the lens much more useful for "normal" photography. You do have to pay for it of course.
• A long macro, say 150mm and longer, may benefit from a tripod mount ring, this makes the entire setup more balanced on a tripod than if you mount the camera body to the pod. The Canon 100mm has an optional tripod mount ring but I've never felt a need for it personally.
• There exist dedicated macro flash units that mount to the end of the macro lens instead of on top of the camera as normal flashes do. The obvious advantage is that the lens can be so close to the subject that it shades the on-camera flash; macro flashes do not have this problem as they are ahead of the lens. This is very far from a must-have however!