I don't agree that it's "wasting a spell slot" to end a concentration spell. Rather, the way I think of it, if you get 1-2 rounds of usefuleness from the spell, it's more than worth the resources spent to cast it. And a "shortened spell list" varies wildly between casters depending on what spells they choose (it only loses 4-5 options if you choose 5-6 concentration spells). I don't see the narrowed selection as limiting, but rather as inspiring creative use of other spells. If I can't protect my allies from my wall of fire, that's not a limitation, that's just a reason to not use it in the thick of melee. Constraints birth creativity, and I've found so far that the Concentration constraint has me looking at old (especially lower-level) spells in a new way.
I feel it is a waste. I've always been a caster that likes to use my resources in a very efficient fashion. I don't like casting spells unless they're needed. 1-2 rounds of usefulness is about all you need in this game. I don't usually get even that. I usually have to cast a protective or utility concentration spell, so that I cannot even use my concentration offensive spells.
I find that tactics largely solve this issue. Bunching up has never been smart, and 5e spellcasters are built to be able to take a wallop or two. A powerful hit you take that disrupts Concentration is one less hit the party healer takes. And if you're casting spells in melee with dragons, well, you're kind of asking for trouble...
Can your spellcasters take a 50 point + breath weapon and a 10 point lair action hit easily? I'm a gnome with a 16 on. I have 72 hit points. AoE attacks and the like chew through them pretty quickly. Concentration checks are tough even for me with a good Con Save.
You don't have much of a choice but to bunch up with mobility as it is. First, spells like
fly and
protection from energy are touch. If you prebuff, that helps. But you still have to stay within a certain range because if concentration drops, you'll have to rebuff your party.
If you spread out, the creature with high mobility starts to pick you off piecemeal. If it is an intelligent creature, it will go after casters first same as an intelligent party would. If you're a wizard or cleric, you stand too far away and you make it so your martials can't engage the creature while it hammers you then flies out of range. It really doesn't care about AoOs from casters. Casters, arcane at least, don't want to use their reaction on an AoO to preserve it for a spell.
A creature that can move 80 feet per round and unleash a 60 foot wide cone can generally get a majority of your party unless you spread out very, very wide. Then it attacks you piecemeal killing the casters.
It's one of those damned if you do, damned if you don't situations.
See, I'm kind of digging it. I'm not paranoid about a failed check. I'm VERY happy to be able to harass my enemies for a few turns, long enough to get their attention like that. I'm finding it makes my choices more interesting.
It's not making my choices more interesting. It's making them less interesting. That is why it isn't fun. I get one concentration spell up and my list shrinks by four or five spells. I've been sifting for non-concentration spells that will be effective and have found few.
Which isn't to discount others' experiences, it's just to point out that there's some disagreement on the issue that points to something other than Concentration itself as the root of the issue.
It's a bunch of factors. Concentration is definitely one of my biggest problems.
I cast
fly. Then I can't cast
slow,
wall of force,
wall of fire,
protection from energy, most illusion spells,
hypnotic pattern,
bigby's hand,
conjure elemental,
conjure minor elemental,
invisibility,
flesh to stone,
globe of invulnerability,
haste (let's be real, this spell is worthless now),
heroism, or
polymorph. It's a huge limitation that takes a lot of options from your spell list. You don't think that is a lot of interesting spells removed from your list of options for casting one buff spell?