Don't know why negative. JB is so much better, even on power saving mode, the live wallpaper transition is smoother, everything seems smoother and faster. The keyboard is better too, plus there is Google Voice Search now.

Driving Mode seems missing from the pull down menu, added Blocking Mode, which is kind of a nice feature.

One more very nice feature, you can finally turn off notifications for each apps/games.
Just got to Applications Manager, tap on specific app/game, you can disable the notification. Hurray~

1. Is there ways for us to join more than 5 contacts-account as 1?

2. Can we add back the "Car/Driving Mode" to the pull-down menu in JB?

It should let user choose. For me, I prefer to manage myself. I don't want to loose something I left half way because the "smart" OS trying to be smart decide to auto-kill it for me while I trying to do something else. or the "smart" OS decide to pre-load some apps in memory (consuming memory) so it launch faster but I may never use/touch that app for few days, I don't mind waiting a while for the app to load and launch, but currently the "smart" OS doesn't let me choose or decide, it pre-loads the apps in memory, if I kill it, it will re-load them.

For general users, it's good enough. But best would be to have more options, to let the user choose the behavior and take responsibility.

Why not? Try typing reply half way, and you switch to other apps and do other stuffs. If there's not enough memory, your browser will be killed automatically without the user knowing, when you switch back, it reloads back the page, altough you're still back at the page, but what you type is previously is gone. It's a common issue, if you have use Android phones with less/lower memory and mutli-task, this happens frequently and is an annoyance.

Pre-loading and Re-loading is not just bloatware and not just Samsung. I used Sony before and I monitor the apps in memory, many apps and games are pre-loaded into memory even you're not running or using them. They do take up memory and once killed you see free memory, but they will restart again. In XDA forums, it's mentioned that this is the OS "smart" way so that you can access your app almost "instantly" instead of waiting for it to "load" from fresh. Personally, I prefer let user to choose, I prefer to decide which app I want to allow.

Yes, for general/most people, the "auto" is nice and good enough. I'm just saying, it'll be even nicer if it let user more choice, to customize and decide which they prefer. I don't want my apps getting auto-killed for no reason or the OS decide to kill the one which is important.
This would be an improvement if available, afterall, Android is more flexible. Would be nice to see this available, just as how we can now FINALLY, turn off notifications of individual apps/games from Application Manager. Now user can choose and decide, which they want to allow and which they don't.

I used before on the Sony Android which only have 512MB ram, totally not enough ram and it's already lacking memory, it still pre-load apps/games in background and can't kill them premanently. I tried the virtual memory on SD card, it's not really a pleasant experience as it's slow and have performance issue.

Before this, I use the Nokia N900 which comes with 256MB ram and 768 Virtual Memory (or swap). When it's using less virtual memory, it's performing really well, but when it uses up more swap memory (e.g. > 100MB), things starts to get less responsive. It's a horrible experience when the app totally non responsive, highly possible it's trying to switch from swap to ram.
The N900 runs on Maemo which is also a Linux based OS, and it doesn't auto-kill apps. Multi-tasking is a breeze and very easy to switch between apps and you can close apps easily too.
Personally I prefer this, but as you have mention, some users may leave the app running and never close them then complains why their device lags or lack of memory, etc. So, it's also good that the OS have the controls. It'll be nice to have choice, let the user customize how the behavior, so for those who prefer to manage their own, can set it.