Recently on the WhyIslam Forum, someone brought up the Convertitis article written by Umm Zaid. I had previously expounded on the article with my own thoughts in two posts, Convertitis Defined and Qur'an Only.
Some brothers and sisters on the forum had commented on how the high iman following their conversion enabled them to begin a very rigorous practice of Islam, and how they viewed that as commendable, even preferable to their present fluctuations in iman.
I know when I converted I got a rush of iman, and made progress (briefly) towards doing things I didn't have the courage to do before. It only lasted about 16 hours before my first real exposure to practicing Muslims, at which point I fell back where I was in the first place. That place was believing in Allah and the Qur'an but somehow assuming I had all the knowledge I needed to really practice Islam, and that I knew more than the other Muslims who were just misguided. I have to say, though, that the longer I have been Muslim, the less I know about Islam. And alhamdulillah. I really look forward to learning more about Islam--the first step has been learning how much I really don't know.
The problem with convertitis is that iman naturally fluctuates, and for those who make Islam a burden right off the bat with drastic changes, they have a hard time maintaining those changes. That's why you see that a person rather than just having high iman and strong practice of Islam, they will actually start "correcting" other people with bad manners, or start pushing Islam strongly without wisdom behind it, just because they don't know any better, and then they get burnt out.
There is the analogy of a piano. If a piano hasn't been tuned in a while, and you have someone come tune it, he'll have to come back the next day and tune it again, and then come back after a couple days, then again after a week, a couple weeks, until he comes about every 6 months or so to keep it in tune. You see, even though you tuned it once, it still tries to go back to how it was, and you have to keep tuning it. I think people are the same way, it takes work to make real change. So when a person might get burnt out, and their iman is low but they have started all these new and unfamiliar practices, it can get tediously difficult to maintain, and ultimately push them out of Islam.
The Prophet (saws) said that Islam is easy, and whoever makes it a rigor it will overpower him.
When I actually started practicing Islam, my constant du'a was "Oh Allah, make Islam easy for me." And alhamdulillah--it was.
(note of reference: I got the piano analogy from Muhammad AlShareef's Fiqh Ad-Da'wah Lecture.)