Hello and welcome!
As usual, Windows 8 is just like any other Windows OS version (except vista) where there are performance and features that are improved. For your multi-monitor setup, Windows 8 has a new feature for that. It's like how Stardock has a program to add a Taskbar and panorama images to each monitor, Windows 8 has that innately built in. You can have different Taskbars, with different items pinned to each Taskbar if you'd like. There are new multi-monitor wallpapers that can span screens better.
The Start Screen can be used in both landscape and portrait mode real well. You can take all you have on your Fences utility and keep your portrait screen as your Start launcher. You can group the tiles, add more, have less, whatever. And in red too!
Windows 8 has MUCH better improvements towards system efficiencies, such as less CPU usage, less disk usage, and overall less RAM usage with fewer processes running. It has a feature called Hybrid Boot that can take a five year old PC and make it boot more akin to a PC with a Solid State Drive in it, like it can boot in 10-15 seconds and shut down in 6-15 seconds. It also has a neat feature called Storage Spaces, which is kind of like a software RAID, but better from what I know. You can allocate different hard drives from SATA to USB to be used as backup media. Windows 8 also has built in anti-virus through the use of Windows Defender, which is like Microsoft Security Essentials, and Windows 8 has actually been found to withstand 85% of the top malware, and has been taken off Kaspersky Labs' list of security riddled software which now is being occupied by an apple product. It may be the most secure Windows version out of the box EVER. Windows Updates are VASTLY improved to the point where it NEVER pesters you anymore about updates, they run in the background, with fewer restarts. If you do need a restart, it'll tell you Windows will restart in like two days if it hasn't been already.
It also introduces these new metro styled apps, as well as a Windows Store for such. The Windows Store is AWESOME! It not only has these metro apps, but since you're a Desktop user, it also houses Desktop apps as well. A few days ago, I found Grand Theft Automobile 4 under the Games category as well as Adobe Photoshop CS6 under Photos. The Desktop apps can be reviewed by people, you can see the details for them, and to download, it takes you to the publisher's download site to get it. The metro apps are pretty neat, especially ones with live tiles. The ones with live tiles are kind of like gadgets from before, except they don't really use system resource to constantly be checking things. You can see the latest headlines, stock info, weather conditions, if you have new emails or new messages from a social network, what song you're currently playing, and you can even use an app to create live picture tiles. They don't really use much system resource, which is nice. If you have a smartphone with certain apps you like to use a lot, chances are that you can get those same apps for Windows 8, in a form to take advantage of more screen estate as well as VASTLY better hardware to render, as these new apps are built to use the graphics card versus the CPU, so graphics and text render more quickly.
Overall, there are several good reasons to upgrade to Windows 8, other than the 40 dollar upgrade fee. I would recommend testing it first, with the Enterprise RTM 90 day trial that brummyfan posted. Make a partition and go. We're here to help, as Windows 8 also takes a bit of time to learn and make it your own.