I suspect that "worst things" will vary according to an individual's lifestyle and will vary throughout his/her life. I have only had a pump for a matter of 6 or 7 months and like the others who have responded would never go back to multiple daily injections. Like Jo, I wish that I had been given the opportunity many years ago but we are where we are.
For me as an 'over 50' , there isn't really a worst thing - it's all good. I know others who have declined the offer of a pump though, owing to the jobs they do - more physically demanding than my desk job. (Saying that, I am sure that there are solutions to all real or percieved problems relating to wearing an insulin pump).
I know that I have become almost obsessive now when it comes to BG testing and trying to get my basal/bolus rates as they should be but then again, when I was a lot younger I paid very little attention to my diabetes, testing very rarely mainly because I didn't have a clue what to do with the readings other than bump up the next insulin injection.
Getting a pump is only half of the story - you get a lot of education and training too and in my view, there is never a "worst thing" about education or training!
Alan