Nymph = medium
Wet / streamer action = slow
For tossing dry flies you don't want to pick a rod that is too stiff. That's a common mistake made by many anglers. Instead of a stiff rod, you want one that has some "spring", or resiliency to it, yet is supple. It's hard to describe in words but the best I can say is that you want a rod that flexes well, but has some resistance to bending. This resistance should grow evenly up the length of the rod.
For most anglers this will be a rod with a progressive action - meaning that it tends to flex evenly & successively more as you move toward the tip of the rod. In other words, this rod bends evenly up it's length. The closer you get to the grip, the more resistant to bending the rod gets.
When choosing a rod for dry fly fishing, it's very important to know your casting abilities. Be honest with yourself here. If you have a tendency to drop your back cast (as most of us do) than consider getting a longer rod. If however, you spend more time fishing in smaller or brush-covered water Than you'll probably benefit from using a shorter rod. Either way, I would say that a rod 7'-6" to 8'-6" long will work fine for a majority of situations. If you choose to go with a longer rod - like a 9' bamboo or say, an 11' graphite switch rod - I'd suggest that you keep the bamboo to no more than a 3 piece & the graphite to a 4 piece. Too many ferrules will interrupt the rod's action causing you to loose some of that nice "dry fly" action.
These are just some of the things to consider when looking for a rod to fish dry flies with. The best thing you can do is to cast a bunch of rods & talk to other anglers or a rod maker too. If you do a lot of dry fly fishing, you're going to want a rod specifically for that. You have many choices, so look around before you buy. Finding the perfect rod is rarely as easy as reading the descriptions in a catalog.