1) Yeah Threat-2.0 and Omen2 in the branches for the 2.4 ptr already uses this extra information and can pull stunts that were completely impossible before. That is per-GUID threat attribution is a done deal which allows proper tracking of AoE aggro for example.
2) Buff and debuffs are trickier, because SPELL_AURA_APPLIED doesn't include a source. I.e. you see that the aura is applied but not by whom. Some spells preceed this by a SPELL_CASTING_SUCCESS, but currently by no means all. Slouken has advertised that an improvement in this department will happen in a future patch (not 2.4 though).
3) I don't know what the status of the tooltip linking is but it's not clear if we'll keep that functionality. In fact I haven't had a chance to test this on the latest PTR build.
Yes, spellIDs are showing in the combat log now and using spellIDs instead of spell names allows for de-localized implementations of anything that involves spells.
Basically one should now always use spellID and only get the name via the spellInfo API when wanting to display the name. Matching really should be done on an ID level. It will then automatically work for any locale.