A few suggestions - hope they haven't already been covered :)

First off, I was thinking that the 'tags' feature is a little harsh on the user. While it's true that the 'Query' box is versatile, it isn't particularly intuitive, in that;
A) You have to know the name of the group you want
-and-
B) Spelling errors can result in either inadvertently selecting or not selected devices by accident.

I was thinking of perhaps a 'tick-box tree-view' mode instead, where each parent tag would be listed [Site 1], perhaps with 'sub-tags' [Core Switches], [Edge Switches], [Wireless APs] listed as children of the parent tag - e.g.;

-[Site 1]
|-[Core Switches]
|-[Edge Switches]
|-[Wireless APs]

This way the user could select which tags they would like to use, in a structured and error-free way.

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Next up, how about a 'summary' or 'description' column in the device list, where the user can add a short note about the device (e.g. No UPS, Located somewhere in my leaky garden shed beneath my lawnmower, etc).

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The "hide big blue toolbar on the left hand side" button is incredibly small, and took me almost four weeks to notice it (maybe my eyesight?). Perhaps putting an [X] in the top right of the blue bar will make it easier for people to see how to hide it.

I presume that at some point this bar will become customisable too?

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I'm quite often asked to change port settings on a switchport for a device, but given no useful information apart from the device's MAC address.

I use the MAC to locate the device using;
show mac-address-table address 0000.0000.0000
-and then-
show cdp neighbour (IOS)
-Or-
Show cam 00-00-00-00-00-00 (CatOS)

While ducking in and out of every switch between here and Azerbaijan, just to locate a bloody switch port is a great time waster, it is also about as much fun as playing the original 'Pong' against a long dead sloth, nailed to a tree on the moon.

Basically, a plugin where you type in an end devices MAC address, the address of a switch and the program scurries off through CDP and high waters to tell me -=BING=-, the switch and switchport the device is on.
Perhaps this would be more of a project for the ZipForge gang, but I'd like to mention that if anybody actually manages to create this plugin, then I... am your bitch.