I dunno - I love gmail's interface. it is fast and responsive, it tracks conversations well... there are a few limitations with being in the browser, but as HTML and browsers develop the shortfalls are disappearing quickly and by being solely responsible for the environment, google have been able to innovate features which they could not do if restricted by the email client.
TAGs are a much better alternative to folders in my humble opinion for email. I use outlook for work - as far as email clients go, it is probably one of the best - I have all my filters and I use categories and tasks - but they are not quite up to par with tags.
The main reason I like tags is so often an email will start off about one subject related to some show and then merge into something else.
In outlook, let's say I get an email related to Les Miserables (my current nightmare - it is still 3 weeks until it tours into my venue and it is killing me), in this example, the subject indicates the contents relates to some venue modification projects, however after dealing with the two projects it then moves on to requirement for an order of water, alterations to crewing and that the production manager has attached the risk assessments to be approved.
I have three options with this email - I can either file all Les Mis stuff in one folder which is nearly unmanageable - my current les mis directory has over 1200 emails - if they were all in one folder I would not be able to cope... I can copy and paste into every sub directory the email deals with (LM2014/Projects/Unistrut, LM2014/Projects/Cement Penetrations, LM2014/Catering, LM2014/Crewing and LM2014/Risk Assessments/Pending) or I can pick the most appropriate folder based on when I think I will need to refer to it. With tags, I would tag it LM2014, Project Unistrut, Project Cement Penetrations, Catering, Crewing, Pending Risk Asssessments. If I needed to find all emails relating to catering for les mis, it is a simple search for +Tag:LM2014 + - outlook, only the second option lets me do that and my outlook data file would quickly be filled as I would have 5 copies of that email (and it's attachments).
3 years down the track when it comes to looking for the engineers certificate for the unistrut, I don't have to remember that it was installed for Les Mis, I just have to search for Tag:Project Unistrut or "Unistrut". I don't have to then click "search all items" and I don't have to wait for outlook to go through every folder searching.