Technical jackets have to keep you dry in nasty weather, breathe so you don't sweat too much when you break trail, and fit comfortably whether you're hiking fast-and-light or wearing a backpack or harness. In addition to being tough, these jackets should also weigh as little as possible to keep your gear weight low when you're headed up the mountain.
Mountain weather can be brutal, so you need top-notch protection from a technical shell. You want a jacket with fabric that boasts a high waterproof rating (20K+) and a highly breathable membrane, ample durability, and a construction that includes fully taped or welded seams.
Because technical shells have to fit comfortably when you're wearing a backpack or a climbing harness, they tend to have a slim to regular fit. Be careful not to choose a fit that's so slim or a size so small that you can't add extra baselayers or an insulating layer.
Technical shells intended for backpacking don't need many bells and whistles, but pockets that are placed higher so you can access them when you're wearing a backpack can come in handy. Jackets you'll wear climbing should also have helmet-compatible hoods and good wrist closures to seal out storms.