A basic treadmill or elliptical, in my experience and to the best of my knowledge, can give you just as good of a workout as an expensive one with lots of gadgets and perks. These added features are usually for interest and convenience and are not necessarily used often, if they are used at all. That said, I find the following things most helpful:
Of course, you will want variable speeds, but I don't know of a model that doesn't offer that, and an incline can help to focus on different muscle groups and intensify your workout. If you are a music listener, many come with iPod jacks and/or speakers. I personally like the monitor and data screens that can control your workout and report your activity in realtime. I also like the screens with body diagrams that show you what muscles are getting the most work. Beware of calorie counters - on ellipticals, at least, they tend to overestimate work done.