Simple.TV
I would love to see a Vevo channel!
There is a VEVO plugin already available in the Channel Directory. IIRC, it is currently limited to US and Canada. If there are other countries with access to videos on the VEVO website, that can certainly be modified.
In the Netherlands the VEVO website and video's are official available but the Plex plug-in is restricted to US-users. :(
If not already requested (Not possible), I would like to vote for Redtube plugin please :)
There is already a RedTube plugin available via the Unsupported Appstore. Check my sig for a link.
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But I am running windows 
But I am running windows :(
That shouldn't really matter for the Unsupported Appstore or the Redtube plugin. Both work on Mac, Windows and Linux.
Sweet. Here I come :)
How about DramaFever.com?
ABC iView Australia.
Flickr authentication support! All my family photos are stored in Flickr as private so it would be great to be able to view these on all the Plex clients (possibly some way of syncing with Flickr too would be nice, so that we aren't constantly downloading it from Flickr)
Either native Trakt.TV Support or an API that the Trakt.TV developer can use (hopefully the nowplaying) I've just seen in .9.8 can do this.
Netflix
Hulu Plus
Multiple ipcamera monitor
Hulu Plus
Netflix
Its missing Netflix - that would be my preferred plugin; however, I guess the Plex team would need to team up with Netflix directly to get proper access as using APIs (as the current plugin and XBMCFlicks does) are far from good solutions.
I Think the ITV Player plugin could use some serious love, as could 4OD
Trakt :)
Sonos support?
Write in vote: More themes. Unless I have missed the place to download different ones. It would be nice to see more themes like with XBMC.
Write in vote: Xbox 360 wireless controller support. It used to work in the old version of PLEX. This may just be a matter of adding the function to the source code of the new version of plex. Kinda like when they added the Harmony Remote Helper which made it so you dont have to setup Remote Buddy to use the Logitech Harmony Remote with Plex.
integration with Trakt.TV