The general problem with how BLM is designed is that it's fantastic on a striking dummy, but throw in the chaos and required movement of a real fight* and it's all too easy to tank your DPS through small mistakes and slight mis-timings (which other jobs don't tend to suffer as much from; e.g. SAM can mis-time things all day and still do good DPS).
And sure, this can be overcome (at least mostly*) though practice and muscle memory, but that doesn't help when you're trying to learn BLM in the first place, especially since the BLM rotation, and thus the timing of everything, drastically changes at key levels... i.e. you can't even really start to learn BLM until it’s level 70. Edit: Or that you basically have to re-learn the timing of your 'movement abilities' for every fight.
* Adding to BLMs issues, it seems like the fight designers have put a lot more emphasis on movement in SB, with certain fights (especially in the 24-mans) having periods of constant and sustained movement, which BLM just isn't designed for (Triple Cast only covers about 8 seconds), and for a 'pure DPS' jobs it just feels s**t basically having to sit on your hands and chain cast Transpose / Scathe while you wait for the boss to stop p**sing about so you can be useful again. /rant
As for what to do about it... well (and assuming they are going to continue to push movement in fights) they could start by trimming all the useless chaff (e.g. Freeze, Sleep) and 'punishment' mechanics (current Enochian :facepalm: ), and giving BLM:
1. Generally shorter cast times
2. More abilities that allow BLM to maintain up-time (and thus high DPS) while moving... Triple Cast is a good start, but how about:
- Enochian = can cast while moving for a short time
- Freeze Time = stops all buffs from counting down / ticking until after your next action (or 30 seconds)
- Outside the Lines = moves your Lay Lines to you
- Duplicate Magic (replaces Scathe after taking another action) = repeats your previous action with no cast time, reverts to Scathe, short cooldown before becoming Duplicate Magic again
TLDR: BLM needs some design changes, as its reliance on minimal movement is no longer compatible with the design of many encounters, and as a 'pure DPS' job this leaves it starkly behind, and simply not fun for inexperienced players.