I know what they are, I know how to do them, etc... I just got one question; what do I need to know to be able to finger them effectively and make sure all strings are sounding properly? is it flexibility I'm lacking? strength? more calluses?

Generally I have no trouble making a bar with my index but I find it very hard to properly finger all strings, especially the first three...
My advice is to do them very slowly and repeatedly, after a while you should be able to finger quickly and effectively.

How I managed to do them was to place my three other fingers alone first and then barre with the index. After you get used to that and get them clean, try to do it all at once.

Barre chords are one of those things that seem tough but once you get them done, they're damn easy.

Hope that helped
I don't usually have trouble with the speed, but I can't seem to get clean notes out of the first strings... obviously, I'm still practicing.
I think what you are lacking is practice. Make sure you are practicing everyday, and they will come over time. If it helps, try using the side of your index finger more, since the side of the finger is more harder than the fleshy part under it the finger.
It's just one of those things that requires loads of practice. Make sure you are doing them right and work at it everiday. Start higher up the neck and move down as you progress. It just takes time...

To answer your question I think strenght is the main factor on playing them but can't tell what's your issue with no visuals of your playig
Keep at it. It's fundamental skill
Theres no silver bullett answer to this , just keep practicing it and honestly , slowly very slowly
it will start to slot in itself , its a combination of building up strength in your fingers that you dont have at the moment and muscle memory , try and use barres instead of open whenever possible , pick one tune and stick at it and remeber it wont happen overnight but will come iyou keep tryni to use it
I think I place it like that too; either that or pointing up. Thanks for the advice everyone... stupid muscles, I can do pushups on my fingers and can't hold a proper barre... :P
bear with you? OH NO, quick fetal position.

lol but in seriousness, its a combination of all of them. ofcourse its important to have calluses, thats something that comes with constant playing. but i think its a mater of strength and accuracy. its important to start slow to get the right position over the fret and to be able to do them smoothly.

i think what you should be aiming for is for barre chords to feel seamless when doing them. i can remember that my barre chords always felt like the strings were too strong. i guess its something that develops over time.