I think there's a simple answer to this: the product doesn't suite you.
That's OK -- I'm sure Xobni's not crying about it.
Somehow you've translated Xobni not suiting you to Xobni not being worth $20 million.
That's OK too, but at the end of the day, there are a lot of start-ups following these threads, and I wouldn't expect any coming your way.
Several people have mentioned the requirement to aggregate across multiple webmail clients - I confess that I'm not affected by this as I consolidate multiple email accounts in one location and avoid the need to check separate email services individually. If operating on multiple webmail clients is common practice, then I confess to having been ignorant of this gap in the market - of course, people could consolidate their email in either Outlook or Gmail.
As for search, does Google Desktop search not work on your PC? It searches both within your email [certainly Outlook and Gmail] and across the filing system, whilst being free. This should go some way to addressing your example of locating conversations. As for the media attachments, I concede that I've long since stopped using my email inbox as a filing system, saving them off to file directories if they are important enough [Pst files in particular always seem to react badly].