Better multitasking, the current one is pretty bad.
More customization.
More options/customization on the notification center.
A native file manager.
Better playlist controls for the music app.
Better tabbed browsing for safari. Chrome seems to have managed to do it better.
A better way to manage your apps. The pages paradigm is cumbersome if you have a lot of apps. How about shortcuts that are on the first page that can take you directly to the 6th page for example).
These are from the top of my head, I'm sure if I actually spent time thinking about this I would come up with more.
There has also been a lot of other people posting ideas and suggestion on what they want on the new iOS, but you choose to ignore them I guess.
I sometimes forget vanilla iOS users lack these features. A jailbreak solves them all.
More Customization: (Not sure what you have in mind specifically) Springtomize can change essentially every visual aspect of ios. From the size of your app icons, to the tint of your screen, all the way to if you want seconds to display on your lockscreen clock.
Notification center functionality: SBsettings is what you need, from my current notification center I can control Bluetooth, airplane mode, cellular, wifi, location services, take simple notes, control brightness, kill background apps, and even reboot my device
Native File Manager: ifile can be used here, i'll link you to the description page because they explain it best
http://moreinfo.thebigboss.org/moreinfo/depiction.php?file=ifileData
Playlist controls: I'm not a big music guy, and i'm not sure exactly what you mean by "better", i'm sure there is something out there for you.
Tabbed browsing: Still not sure what you mean by "better", but there is one hell of a lot of tweaks for safari. I personally prefer chrome (for which you can add downloading capability and other features) so I don't screw around with those tweaks either
Pages: Not sure if there is a tweak for this specifically, but you can make it so pages have the ability to scroll vertically(Springtomize), have no locked grid (put apps anywhere-tweak called "gridlock"), make it so folders have pages and use the full screen (tweak called "FolderEnhancer"), even customize the scrolling animation between pages (tweak called "Barrel")
And I almost forgot multitaking, for this you have a few options:
Tweak called "Backgrounder" gives you true unpaused backgrounding. It can be tough on RAM (which is why apple doesn't imlement it), but if you want apps to continue running as if they were active, but while in the background, this tweak does it.
Another option - tweak called "Quasar" This one is best explained via video.
http://www.youtube.com/watch?v=7cYWha8YFi8
It's actually very light on RAM (which you can monitor in real-time with SBsettings), but comes with a hefty price tag ($10)
Not saying jailbreaking having these features excuses apple from not having them, but you can't say that you don't have a choice when it comes to functionality of iOS.