/Castsequence must wait for acknowledgement from the server of a completed cast before it casts the next in the sequence, where /cast doesn't care if the original spell went off or not.

this is the same reason that if you have a cooldown spell in a /castsequence that the sequence won't go past that spell while it is on cooldown.

here is an example.

on my mage i have this:

/use 13

/use 14

/cast combustion

/cast fireball

this allow me to spam this key and it will fireball until i run out of mana, works great. uses trinkets every time the cooldown is up, uses combustion each time the cooldown is up, and when they aren't, just spams fireball.

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if i were to change it to:

/use 13

/use 14

/castsequence combustion, fireball

it would cast combustion and one fireball, then get stuck trying to cast combustion until the cooldown is up. it would not "bypass" the spell on cooldown. therefore, /castsequence waits for a successfull cast before moving to the next one in the list where as /cast does not check, it just keeps running down the line.

where this becomes a problem is that you can start casting a spell very briefly before the last one is finished (this is why mods like quartz show latency, so you can cast a bit early). with /cast you can cast as soon as the client believes the spell is done, but with /castsequence, it waits until the cast is finished, verifies from the server that the spell was successful, then moves to the next one. (i may have gotten my client/server operations mixed up as i don't know where the operations really lie, either on a server check or client check, but that really isn't the point and the logic is relatively sound).

i hope that answers some questions. switching to straight /cast macros really increased my dps by an order of magnitude.