Without a doubt, in my experience the #1 single biggest mistake I see beginners make is allowing slack in the line.
You can have the sharpest hooks, the right offering, be in the right place at the right time and do everything else right, but if you have slack, you've got jack.
Slack is a negative to every aspect of steelheading regardless of technique or style and yet it is the most common fault you will among novices. From beginning to end, slack negatively affects presentation, the ability to detect the take, the opportunity to strike or set hook, control during the fight and success in landing a fish. Take the slack out and you not only land more but you will actually hook more too.
Many other things are important to becoming a more successful steelheader but ultimately it is line control that separates the expert from the novice. Once a novice becomes expert it is possible to use slack with control in certain techniques, but even the expert tries to avoid this at all costs because they know it puts them at disadvantage.
I believe it was the General I once heard say, 'take the slack out of your line and you might start catching some of those fish you've been missing'.
I think my response at the time was probably, 'what fish?'.