Anne,

You are absolutely right, yoga is everywhere and at the same time nowhere at all.

I believe the different types of yoga are, to one degree or another, a function of western culture's need for constant stimulation (and thus a new toy every few years), and a necessary diversity needed to suit a dynamic, growing process called "yoga".

In my training as a yoga teacher we were required to investigate and report on all of the different types of yoga flavors. Here are some differences based on my study, practice, and experience:

There are practices that are warm, hot, and downright sweaty. There are those that move to something else on each breath and those that stay in one position for many breaths. There are practices that teach only asana and others that teach all of the Eight Limbs from Patanjali's Yoga Sutras.

Many of the current "styles" qualify as "hatha yoga". Some of the practices have strong connection to a lineage dating back thousands of years while others do not. Some practices take this from here and that from there and mix in someone's opinion, preference or personal experience and then slap a label on it and call it Anne's Yoga or Shanti or Lotus or what-have-you.

I believe the larger question is "what yoga is for me?" And since you have free will you are never over your head. You simply do not return to a class that does not serve your growth as a human being.

Relative to expectation, there is no reason (or benefit) from expectation. You'll actually reap a greater harvest heading in without any expectation at all. But if you must have such a thing for peace of mind, then I can post again since I originally trained in the power vinyasa practice.

There is classical yoga and then there is classical yoga, As Gordon states, style is just a Western ideal.  Though one can grow in a yoga class, yoga really begins outside of the class.  Choose a class that enables you to go beyond the mere walls of the studio (please remember fitness centres offer a strange version of 'yoga' its not yoga) and beyond the asana, yoga for me is practiced 24/7, its a way of being, its a way of interacting with the world, nature and who I am.  Find yoga that enlightens you to who you are.