The main effect of the Gmail success is that the biggest companies that
offers web-based email systems (like Microsoft and Yahoo) are working to improve
their tools.
Today, Yahoo! announced a limited beta test (invite only) of
its new Web-based Email
tool, a preview of what will be the next generation email system
for the company and Charlene
Li's Blog has a good preview of this new system.

What I want to put in evidence here is that there's a
clear direction for the Google's competitor to fight the Gmail success: making
the Web-based email systems to resemble more of a client-like experience,
give to the users a system that is totally similar to what they can have on
their desktop.
This is undoubtely in my opinion one of the target that all the web-based
email client must have, but keeping in mind that there's also another extremely
important aspect to consider: the performances.
Gmail has a simple user interface but performances are really good, also with
lots of Mb of emails. An email system with a rich user interface must have the
same performances, quick to open and to work with emails, to retrieve them and
to delete them. When perfomances and rich design will go together, we'll
have the perfect email system.
P.S. another little consideration: the new Yahoo! interface (and the future
Hotmail too) is really similar to Outlook... casuality or Outlook UI is now a
model? 