This week I played a new character in a Dungeons & Dragons campaign, a bard of the college of lore. That turned out to be not so obvious, because that build has lots of spells that require concentration. In 5th edition D&D, concentration means you can only have one spell like that ongoing at any time, and if you get hit you need to do a saving throw to keep the spell up.

The first restriction is sometimes annoying, but in my opinion well-balanced. Those flying, invisible mages of previous editions are no more. My bard has greater invisibility, which allows him to remain invisible while attacking or casting spells. That is powerful enough, even if I can cast only non-concentration spells like lightning bolt while invisible.

I am much less happy with concentration breaking on a failed constitution saving throw when taking damage. If you look at a spell like Confusion, the enemy gets a saving throw when I cast the spell on him, then he gets another saving throw every round to break the effect, and the effect also breaks if I lose concentration due to getting hit by something and failing my save. That is a lot possibilities to negate a spell effect. The only class doing a bit better here is sorcerers, who are proficient in constitution saving throws. Other classes would need to take the war caster feat to improve their chances of their concentration holding, but that comes at the cost of not taking ability score increases.

In some cases the necessity to keep concentration up severely limits the options of a spell. For example I could theoretically use Polymorph to transform myself into a giant ape and kick ass in melee combat. However in melee I would lose concentration rather quickly, especially against enemies who have more than one attack per round. So the next best use for the spell is to cast it on the group's wild shape druid. But then I have to a) stand back in order to not get hit, and b) not use any other concentration spells while the druid is going ape.

On the one hand I do like to play a caster that doesn't simply blast enemies with direct damage, but rather works as a support character, helping his allies, and hindering the enemy with various effects. But to some extent I feel that the D&D rules on concentration somewhat discourage this style of play, and make throwing fireballs comparatively far more effective.