I was never really Buddhist, but I did ascribe to Eastern religion/philosophy. I was leaning closer to Hinduism than Buddhism, but really I was neither, and more believed in a eclectic blend of things. I will say why I left though, since that’s relatively close to Buddhism.
The biggest reason is the culture barrier. The religion is not very convert friendly, and many of them are actively hostile to converts. This alone put me at unease. If one believes that the religion is the truth and one benefits from following it, wouldn’t you want your religion to spread? I realize they don’t believe in steep penalties for error (if you mess up, you’ll be reborn to try again), but still.
Another thing, which is very similar to the reason I left Protestantism, is their lack of an official stance on any one issue. If you did this, that’s ok, but you can do this too. You can believe or do almost anything.
They’re concept of the afterlife turned me away too. For one thing, the concept of justice is to be reborn in another life, so you can learn from your errors. But since nobody accurately remembers their past lives, your memories get wiped every single time, so not much learning going on. And the ultimate goal is to escape the life/death cycle and go into nothing-ness. I found fading into nothing after death to be terrifying, close to some Christians’ interpretation of Hell, not as some goal I should be working for.
What I did like about it though was the meditation aspects and the eternal search for truth. However, now I see that while they may be searching for the Truth, the Catholics have it handed to them.
Another thing I did like is the fact that their idea of sin is actually very close to that of Catholic, even if they don’t agree on what is and is not sin. The Buddhists lack a final judgement deity, and so they see sin as intrinsically harmful and will naturally carry you into whatever life you may live. Similar thing with the Catholics, sin isn’t evil and pave the road to Hell because God arbitrarily deemed it so, sin does that simply because of the nature of sin and that’s what sin does. Jesus Christ wipes away our sin though, as long as we are repentant and accept Him.