The answer is weaved amongst the various posts here, I'll so my best, but it looks like a really simple and obvious reason.
1) is that there usually is a "user" variable in the udev rule some thing like USER="your_user" probably right after the GROUP="plugdev"
2) You need to use the correct SYSFS{idVendor}==”####″ and SYSFS{idProduct}=="####" values for your device/s. If you have devices from more than one manufacture, say like one from Samsung and one from HTC, then you need to have an entry(rule) for each vendor, not an entry for each device but for each different vendor you will use, so you need an entry for HTC and Samsung. It looks like you have your entry for Samsung now you need another. Remember the USER="your_user". Use 'lsusb' like Robert Seimer suggests to find the idVendor and idProduct, they are usually some numbers and letters in this format X#X#:#X#X I think the first one is the idVendor and the second idProduct but your going to need to do this for each brand of phone/tablet you have.
3) I havent figured out how 51-adb.rules and 99-adb.rules are different or why.
4) maybe try adding "plugdev" group to your user with "usermod -a -G plugdev your_user", Try that at your own risk, though I don't thinks it anyriskier than launching a gui as root but I believe if necessary you should at least use "gksudo eclipse" instead.
I hope that helped clearify some things, the udev rules syntax is a bit of a mystery to me aswell, but from what I hear it can be different for different systems so try some things out, one ate a time, and note what change works.
