Of course you can't modify poses and stop during the middle of an exercise, it's a disciplined practice. This isn't a free for all. Maybe it's just the trained ballerina in me that knows what it takes to suffer through a 90 minute workout, but why is everyone who's given a negative review such a weenie? You're asked not to leave the room because it's too hard on your body to keep adjusting to the temperatures. Not to mention the fact that you're making the room cooler for anyone who happens to be near the door.
The instructors are there not to single you out during your first class, but to encourage you to try and persevere through. The studio is a lot nicer and way more clean than other hot yoga/Bikram studios in town. The instructors are all friendly, and if you can't take a few corrections, take your ass back to the YMCA.