when you click a note, FL remembers that note length for subsequent note entry. editing velocity or other parameters is instantaneous. lengthening/shortening/slicing notes are quick processes. left click adds note, right click deletes, scroll wheel moves and zooms. Here's a post from a user in the thread as an example:
I love ableton to death and don't see a reason why such a great company and software shouldn't also have the best piano roll.
This post isn't for a single feature, but a plea to look over the piano roll as a whole and make changes necessary to equal or better the top piano rolls on the market. If you open that thread and cmd f search piano roll, it seems that every person to bring it up favors the FL piano roll.
The piano roll is such a vital part of a DAW and the song writing process that I see no reason for a software like Ableton to really make it the best available. I believe if Ableton put time into matching this experience, it might not just convert FL users, but keep many. It seems that a lot of people agree that the Piano Roll in Fruity Loops has a much better User Experience. Here is (what I think) is an amazing reddit post asking people who have switched between FL and Ableton to describe their experiences with the two, what they missed, what they like etc.
