The first thing to consider is where you're going to be carrying it, and under what circumstances. The (optically) best scopes are heavy and bulky and need a solid tripod; they're not ideal if you're hiking up a mountain with camera and day hiking gear as well. Travel scopes (I've got a mighty midget) have good optical quality and are smaller and lighter, and work well on a basic carbon fibre tripod.
The size of the front element is mainly important if you want to use it at dawn/dusk, or if you want to take photos through it. Bigger means more light gathering power, at the expense of weight again.
The magnification is set by the eyepiece. A zoom eyepiece is very useful, especially on a tripod, when you can zoom out to find your subject, then zoom in to see it clearly. You can usually change your eyepiece for different situations. Cheap scopes and their eyepieces often suffer from chromatic aberration (coloured edges) and general loss of sharpness when zoomed in. A light scope at up to about 15x magnification can just about be hand held, but any more than that and you'll need a tripod, bean bag or similar. I like a tripod with a simple ball head that steadies the scope while allowing me to scan freely before locking off. This is for wildlife and occasionally stargazing.
Angled scopes are common, so that if the scope is looking horizontally, you look down at 45° into the eyepieces. This is useful if you want to look up, and in hides if you're tall, but isn't necessary if you're looking for wildlife on the ground using a tall tripod in the open or seawatching from the top of a cliff.