Let's start this off by clarifying—I love Star Wars. When I was small, most little girls pretended to be princesses, and so did I, but it was never a prince who rescued the princess... it was a Jedi. But, the older I get, the more I realize the main problems with the Jedi Order, and why it fell eventually.
1. The Jedi Order doesn't have a way to support themselves.
The members of the Jedi order are comparable to monks, in that they all live together in a Temple, and in that communal home, everything is provided for them. However, there is no perceivable means of income for the Jedi Knights, and so the people of the galaxy, who are obviously already struggling to stay out of poverty, must be forced to pay taxes to support the Jedi Knights. Additionally, the Jedi don't live in the simplest way possible, so the citizens of the galaxy have good reason to be resentful when they are struggling to pay for food, and yet still have to pay to support something as extravagant as a full Jedi Knight library that the citizens are unable to access themselves.
2. There seems to be no scouting system to find those strong in the Force.
As far as I can tell, there seems to be no active Jedi outreach to find those in poverty or slavery who also happen to be strong in the force. In order to be "tested," you must go to the Jedi Order itself, instead of the Order coming to you. On the outskirts of the galaxy, there is no way for people to know if they are strong in the Force or not, since there is practically zero professional Jedi presence there, unless you happen to have Ben Kenobi hovering nearby. Doesn't this mean that in order to be a Jedi, you must be financially well-off, since there is no way for a slave or person in extreme poverty to escape for long enough to discover his or her true potential?
3. The Jedi Way of dealing with emotional trauma is atrocious.
Every time I watch Star Wars, I always end up yelling at the TV because the Jedi Order is SO emotionally oppressive. I used to think I would make a pretty good Jedi, but, now, the more I think about it, I would definitely end up going to the Dark Side, because at least the Emperor offers a healthier way to process tough emotions! Yoda doesn't let Luke mentally process any of his grievances, which is probably why Luke is even tempted to go to the Dark Side in the first place. If the Jedi Order had thought about the long term effects of emotional development and balance, then they probably wouldn't have had so many problems with so many powerful Jedi going off the deep end and holding onto a poisonous personal vendetta against the leaders of the Jedi Order who taught them to keep their feelings down and never deal with tragedy.
