1a) Well I guess it all depends where you draw the line. For example, technically sword affixes that boost Fire Magic attacks or induce a Fire vulnerability aren't completely useless because of that one fireball that goes on your Special Abilities bar. Nonetheless, when I see those affixes on a sword item I go "%!&#!$" and consider the item to be junk. This may be less of an issue later on when items can be reliably sold for full vendor value, or salvaged for valuable components or what have you, but right now it just fills me with Internet Rage.
1b) Given that animal curses are (I think) supposed to be long-duration non-swappable effects, would it be too generous to condense all animal form gear affixes into a single set that then changes depending on what form you are in e.g. the Cowl of Animalistic Awesomeness drops, it has 2 animal form affixes. To a non-animal player it will say something like "Your basic animal attack does extra damage" and "One of your animal special attacks has a chance to peform an additional effect". If I'm a spider, it will say "Your Spider Bite attack does +X damage" and "Your Inject Venom attack has a Y% chance to be an AoE". If I'm a cow it will say, "Your [whatever the name of the cow basic attack is] does +X damage" and "Your [some special cow attack] has a Y% chance to do double damage." Werewolf affixes might have to remain their own group because they "stack" with regular animal forms.
1c) Skill proliferation will increasingly make getting a set of gear that boosts particular skill X difficult. This could promote a nice player economy, however, if there will be some sort of auction house or player vendor system. It's interesting how the NPC vendors partially fulfil this role currently (although sad how the items are wiped each patch).
2a) Right now, I prefer not to group as there is no formal grouping system (can't see party members, chat to them in party chat, see where they are etc etc), and a party member dying can mean a lot of reclearing as there's no way to revive them. So bit of a catch-22 there. Also rage-attacks are increasingly hard to mitigate with multiple people attacking a mob; a nice change might be an emote/animation/screen flash when the mob is about to unleash it's rage attack, followed by a short delay, in which you can use a rage reducer if you're quick enough, and if you fail to do so the rage attack goes off. Finally, I do think the loot situation needs tweaking - it should be an incentive to group (beyond the fact that you might kill something you can't kill on your own). If all party members could skin and autopsy the same mob, for example, that would incentivise grouping a bit.
2b) I actually think Mentalism is in greater need of buff-wiping - as the Golem already despawns when Battle Chemistry isn't one of your active skills, you lose a huge chunk of Battle Chem's power if you just buff-and-swap.
3a) Synergy buffs of the form you suggest there don't sound too strong to me - attacks of Skill X increase damage of school Y sounds like a good sweet point, especially as it can promote ability priority changes. There's 10+ damage schools already (and maybe more to come?) so it's still fairly narrow.
3b) I've seen some of the new Mentalism affixes - the main thing that struck me was how some of them were a tiny chance for a ridiculously strong effect - I think it was a 2% chance to restore 250 armour (which is pretty much your entire armour pool, unless you're decked out in top-level gear). Personally I prefer a bigger chance for a more moderate effect, but that's just me.
4) I'm hoping that if an item gets budgeted 2 powers rather than 3, then those 2 powers are "stronger" than if there were 3 of them on the item e.g. (+3 to attack X, 50% chance for Y to AoE) vs (+2 to attack X, 33% chance for Y to AoE, attack Z returns 5 power on use) ? Cos otherwise we're back to crappy blues vs stronger blues, which was my original concern. Also, you may wish to similarly increment the code for purple items, as it's currently possible to make epics with just 2 affixes.