Because WotC no longer allows new subscribers to the Character Builder and Compendium, which excludes almost all of my players, and because those sites are becoming glitchier with each passing month, I have finally decided to throw in the towel and switch to 5th Edition.
Welcome aboard, though, really, that's not a very good reason. You shouldn't feel 'forced' into it.
So, I'm not sure if this has been pointed out yet, but the 4e tools are accessible again.
And despite it being hidden, it DOES seem that you can renew a subscription (though you cannot start a brand-new subscription if I'm not mistaken). You can click "manage subscription" on an order page, if you have any orders in your Order History. From that "manage subscription" link, there is a renewal option.
I'll have to try that. Where is this 'order page?'
The Character Builder is WAY more convenient than searching through all the books: plus my players do not have all the books. Likewise the Compendium for rules. This is why I avoided 5E since it has no convenient Builder. On the other hand, there are few books for 5E yet, so less wading. With the on-line tools almost defunct, I may as well just use the 5E books.
The 4E tools work for me since I am a subscriber (although I find the search function on the compendium extremely buggy), but my new players cannot make a new subscription.
One option is to use pre-gens for new players. Whether you're doing that with the 5e books or the 4e CB, it reduces confusion.
That said, I am looking for advice from other dungeon masters and thinkers who understand both 4E and 5E.
1. My first question: is the problem of the 3E quadratic wizard versus linear fighter back again?
Yes. But, not as bad nor as overtly. In older eds, all a casters spells got better as he leveled, and he got more of them - thus 'quadratic.' In 3.5, lower-level spells topped out in damage and saves vs spells got better with slot level, not caster level, so while the caster got more spells, there were limits to how much better the lower-level ones got at he leveled up and his top-level slots were most important. In 5e, spells scale mainly with slot level, but saves vs spells scale with character level (not even caster level), so top-level slots are, again, most important, but you're still getting more and more powerful spells as you level up. So the wizard may still be 'quadratic' (you've still got x-squared and xy in there), but they're not as dramatically so (a and b are lower values).
Do I have to prepare for spellcasters trying to break my encounters?
Not so much, no. There's not a lot of point to being 'prepared' in that sense - there are encounter guidelines but they're very much just guidelines. Your most powerful tools as a DM are rulings, not matching PCs powergamed-monster-for-powergamed-build nor wielding the ban-hammer.
If a player pulls something that turns out to be broken, you rule against it right then and move on. Later, you can let it work in a situation where it wouldn't be broken.
2. Second question, for other 4E players who have switched over: Which rôles do the new classes approximate? (I notice for example that the PHB ranger is not nearly the Striker of 4E, but perhaps no class is?)
They don't, really. Formal roles are gone.
Most classes can deliver a lot of DPR with the right choices, and several are arguably Strikers in the sense of being all but hardwired to do DPR and little else (Fighter, especially, but also Barbarian and Rogue). Rangers and Warlocks can do high DPR fairly straightforwardly as well. Neo-Vancian casters with the right spells on their lists are leaders (Cleric, Druid, Bard) or controllers (Cleric, Druid, Bard, Wizard), or even strikers ('blasters') based on what spells they prepped that day. Sorcerers and Warlocks are controllers or blasters depending on choices made at chargen/level-up rather than at long rests.
The more traditional/de-facto roles could be said to be:
Tank (DPR and high durability): Fighter, Barbarian, Paladin, and, oddly, 'Moon' Druid.
Heal-bot: Cleric, Paladin, Druid, Bard, even Ranger, a little.
Mage (solves any problem with limited-resource spells): Wizard (in spades), Cleric, Druid, Bard, or, less so, Sorcerer, Warlock, Paladin, Ranger, AT, EK, and even, much less so, Monk, 'Totem' Barbarian, Magic Initiate and/or Ritual Caster feats.
Thief (scouts, opens locks, gets killed by traps, is generally expendable): Rogue, to a lesser extent Monk or, in the woods, Ranger
I guess we could add slightly less-traditional:
Blaster (range and/or area DPR): Warlock, Sorcerer, Wizard, Cleric, 'Land' Druid, Fighter or Ranger archery style w/Sharp-shooter feat.
Opportunist (situational DPR, good skills): Rogue, Monk.
5th-Wheel (doesn't do any one traditional role all that well): Monk, Ranger.
We could come up with others. Ultimately each class is not designed to make a general range of contributions to the party as part of a team, but to evoke the feel of that class in the classic game (or 3.5, in the case of the Sorcerer & Warlock).
Rather than expecting the players to come up with a balanced party, you just need to adapt challenges to the party - or they have to adapt their strategies to the resources they have, which could include declining certain challenges....
By the way, our sessions do not typically involve dungeons with room after room of traps and monsters. The party has yet to strike off into an entirely hostile region with no places to recover. I think the suggestion of a night for a short rest and a week for a long rest sound appropriate. Or some equivalent of milestones as in 4E.
If you have any sort of consistent pacing to your campaign, you can just simply adjust the definition of long and short rest to suit. Even if you don't, you could come up with something more arbitrary, like 13A's 'full heal up' and 'campaign loss' concepts (see 13th Age).