Most of my beta playthrough was Sorcerer, as geared as I could for my flow. Which was mostly Hydras, Fireball, Fire bolt, Meteor/Fire Wall and fire ultimate, of course.
While gameplay was mostly enjoyable, I noticed often how I constantly was missing on casting abilities, because other one was mid-cast. I’d cast fire bolt, and TRY to cast meteor, squint at the mess between monsters, see nothing happens, and then cast again, frustrated. And that happened more than often, and was constant thing I had to keep in back of my mind to ‘optimize’ my flow.
But it was just annoying, really. If I have mana and it’s off cooldown, I want to cast it. But it felt like I was fighting this “global cooldown” constantly, and it was not really clear how it works.
It was especially visible while trying to cast-chain on the ultimate 8-sec window with no mana cost. It feels very messy, not as satisfyingly as it could be.
I’d really like either clear UI feedback of the cooldown, or some tweaking of speed of it. It would be amazing if you could tweak it yourself with itemization as well. Spell haste % on items.
It’s an interesting point, I did notice the free mana time for Incinerate as an example, was more inconvenient than useful. Although it could only be less useful at is right now since we don’t have all our SP and access to paragons that could very well have what you want in it.
I hope that’s the case. But so far I didn’t notice an item affix that speeds it up, so it’s still not clear.
To be precise, there were some affixes speeding up Basic abilities, but not often more than that. Either way, it was not very clear on the UI if I could cast next ability or not. In WoW for example, you do get an UI swirl before you can cast anything else, AND there are abilities that are instant. While I don’t like completely the global cooldown in WoW, it is at least clear how it works. Less so in D4.
I noticed the same thing with what appears to be global cooldown after cast, keep in mind that the paragon system “might” play a role in lowering global cooldown but for the answer to that we have to wait and see 
I dont like the HUGE CDs on pretty much any skill. But yeah its the beginning and beta and so on…wait and see and hope. and blizzard for the love of lillith praise be let us SKIP conversations PLEASE!