Blue
Ravenous Bugblatter Beast of Traal
Q1: What are you doing to buff the martial classes to balance out the increased power and action economy of spellcasters?
Q2: What situations are you putting in the game that are much more difficult due to no Concentration spells?
Q3: Are you using opponents with Concentration spells against them, so they see the power and utility of those effects rather than just the limitations?
Q4: Does it come down to a few spells, that are nerfed too much due to Concentration, that might be balanced enough with a bit of surgical homebrew? (Looking at you, Flame Blade.)
I can only give [MENTION=16728]schnee[/MENTION] XP once for this post, so the only other way I can agree how important these questions are is to quote them and talk about them.
This definitely increases the power of casters. Are you planning on increasing the martial and monsters, or putting a corresponding debuff on casters?
If the debuff is only on when concentrating on more than one, it's not enough because the character has 100% of the power they had before, plus more options. There needs to be a debuff that occurs outside that time as well. (It could be something that lingers after use, that's still outside the time of use.)
Maybe losing concentration also causes 1 HP of damage per spell slot level as it grounds out in the caster. Not punitive, but a minor debuff for all casting and one that is a bigger deal with concentrating on more than one.
If there isn't a debuff, then we're looking at buffing martial characters (and to some degree half-casters), and then bringing foes up to snuff with the new, mightier PCs.
On the other hand, Q4 points out correctly that there are some spells that Concentration is just punitive on and a bit of rework could correct. If that's the major issue.