Ok, let's get this sorted: what can and cannot be blocked?
[*]s/s passive, Deflect Bolts: Block 15% additional damage from projectiles and ranged attacks
[*]s/s active, Defensive Posture: Activate to reflect next spell projectile at attacker
[*]resto staff passive, Absorb: Blocking spells restores 5 magicka
[*]Mages Guild passive?, Greater Ward?: reduce incoming spell damage by 15% while blocking
The last one on the list seems rather confusing, it actually doesn't exist on the mystical google doc spreadsheet, which is probably most up to date, but it does exit on the leak we got in news here, which seems to match the Quakecon build. If that passive no longer exists, that would be actually convenient for my hypothesis, since it's the only blocking related passive that doesn't apparently affect block as a mechanic itself, but rather gives you damage resistance while blocking.
Wording of it all can be rather tricky, especially given they are just leaks and things can get lost in translation, but both s/s skills listed talk about "projectiles", and Defensive Posture explicitly states "spell projectile" (both on the spreadsheet and on Tamriel Foundry listing).
With all that in mind my hypothesis would be that blocking any kind of a ranged projectile - be it arrow, a destro/resto staff light or heavy attack, or any sort of a fireball-ish kind of a spell - is possible. That means, ESO would need to use a mechanic separating offensive spells between projectiles and non-projectiles.
Also I'm now tempted to make room for Defensive Posture in my build, goddammit.
The real question isn't if you can block them. It's, "If you block a powered ranged attack, will the person be knocked like they would be knocked if they're using melee? Does this apply to Resto Staff which uses a magic attack, albeit at short ranged? And, if it applies to Resto, does it also apply to Destro and Bow if they're within countering-range (melee range)?"
PVE I'd take Defensive Posture and Puncture for sure, but I don't think a single spell is worth reflecting in PVP for the price of their spell-cast. In a game where there's no big cooldowns that you can expect and there's a lot of small spells, reflecting spells will be much better over-time, (Reflective Scale//Eclipse) than for a single spell at a time, unless you have a way of knowing when someone's going to cast an Ultimate or something. In PVE you'll probably have a better idea of when something worth reflecting is coming your way and you can be prepared for it. In PVP, will anything be worth the cost? Will everything be worth the cost? Can you make a 5 heavy 2 medium S&S Build that's able to continuously spam it and it can beat any caster in PVP? Can you do this with any MP heavy Temp or DK, forcing the opponent to just stop using spells on you entirely? Maybe these abilities don't even work in PVP, but I don't see them doing that.