Attributes adds damage instead of making it that damage. Example: Diamond sword does 7 damage in 1.5.2 but with the attributes, it does +7 damage, in addition to the one from fists, so now does 8 damage instead of 7.,

Yes, that's intended. That's what "+" means; "plus", "added" damage.
https://twitter.com/Dinnerbone/status/337542221177954304
According to Dinnerbone, this allows for boots that speed you up, bows that slow you down, armor that increases health directly, ect.
https://twitter.com/Dinnerbone/status/337540303647027201

He has already said this may break previous maps slightly, or not at all, since it's only dealing a single heart more damage, it all depends really.

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Confirmed.

Eh... I don't really like this one... Confirmed anyways...

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.