we have a large number of users using roundcube now, and two complaints kept coming up again and again:
message list view with the preview pane disabled
(setting a new paragraph instead of a newline)
both of them can't be "fixed" for everyone, as a fixing it for one group (the ones who complain) breaks it for the other group (those who like it as is). instead i've implemented them locally as new per-user config options, allowing the users to override the default behavior for each.
as both needed some changes to some of the javascript files they aren't suitable candidates for simple plugins. are these two options things that would be of use to the general project? if so, i'll submit a couple enhancement tickets with pull requests with the changes for them. (i'm fuzzy on how this type of feature enhancement should be handled)
for the single-as-double click option, when it is enabled (disabled by default) shift/ctrl-clicking still selects messages, single clicking just switches from selecting the message to opening the message.
for the html editor enter behaviour, when it is enabled (disabled by default) the behavior of the enter key switches: enter adds a newline and shift-enter adds a new paragraph (the inverse of how it is now). via this tinymce option: http://www.tinymce.com/wiki.php/Configuration:forced_root_block