Weight rating.
As light as possible, can't believe how heavy my borrowed one has made my bike feel!
I meant in terms of how much weight it can carry. Most are good for 20+kg, though, which is plenty for touring, but might be an issue for very heavy shopping.
Decent dog-leg to stop pannier flapping about.
Not sure what that means, I've been borrowing a rack and ortleib panniers (which is what I am ordering) seem easy to use.
Basically, how wide the rack is at the bottom. Not really an issue with Ortliebs, as they have a nice rigid plastic structure to them, but floppier fabric panniers may benefit from something to keep them away from the wheel.
Rail diameter and invalid hook positions compatible with racks/panniers you already have.
Help!
The diameter of the rails doesn't matter in terms of performance, but many panniers have hooks that work better (more secure, or just less rattly) with some diameters than others. Ortliebs come with plastic inserts for the hooks to adjust for this, so will hook securely to pretty much anything. If you have racks on other bikes you want the diameters to match, though, as removing and re-fitting the inserts is a complete bastard of a job.
As for hook positions, if the rack rail has struts welded to it, forming a 'T', the pannier cannot hook over that particular bit of the rail. Most panniers allow their hooks to be moved about horizontally to adjust around them (The fittings used on Ortliebs are particularly good at this), so again it becomes mostly an issue if you want things to be easily interchangeable with other racks/panniers you might have.
Something you can hook a strap/bungee under/through at the bottom end.
?
Usually a loop or a inverted-V-shaped bit - just somewhere obvious low down that you can hook a bungee cord on to. Not something I make a great deal of use of, except when using a set of cheapo panniers that need something to attach a strap to at the bottom end to stop them flapping about (Ortliebs have a hook that goes behind one of the rack's vertical struts for this). Some people like to use bungee cord to hold things to the top of the rack, and you can get sets of multiple-bungees-on-one-set-of-hooks for the purpose that hook to something low down. (I tend to only bungee things across the top of panniers, so hook them to the front and back of the rack.)
