After almost three-hundred sessions both DM'ing and playing as a PC, I can confidently pick out what I like and dislike about the 4E system.
The Good - Overall I think the designers did a brave, incredible job creating an innovative rules set that reflected almost exactly how my D&D actually played at the table. Yes, the Fighter did get bored being a collection of feats with legs. Yes, the Wizard did get pissed off when she ran out of spells. And yes, my Cleric was frustrated being nothing but a heal-bot at high levels. Those things were all true and they were all fixed in 4E.
On top of that, with a selection of by-level difficulty guidelines which made improvisation an absolute breeze, and a framework for encounter building that left all the previous editions looking neanderthal in comparison, the game was a huge breath of fresh air in a 3E world with which I'd become increasingly disenchanted.
Team building - Watching PC's at work on the battle-mat and then manoeuvring my own dudes in strategic response is one of the great pleasures of the 4E game. This didn't come without its cost (see 'The Bad'), but when the PC's got a plan in their heads and were then able to execute it (a far cry from previous editions), it was glorious. Just, glorious.
At the heart of this success story are combat roles. There was a lot of noise at the time about pigeon-holing PC's, but in the spirit of designing the game around the way it was actually played, the concept of the plate-armoured tank, the glass cannon clothy, the tricksy damage-dealer, and the selfless support class fit neatly into D&D precedent and worked very well at the table. Defenders in particular had well-designed tricks that made their job incredibly rewarding.
The same applied to monsters and villains. They had their own roles in combat, which meant building effective, challenging encounters boasting a variety of dangers was a quick and easy job. This in turn lay down a broad canvas for PC's to strategise and make the most out of their shticks.
Encounter building - With monster roles as a starting point, encounter design was already off the blocks running. Terrain powers created an even more sophisticated encounter-building grammar. But what really set 4E off in this regard, and something which could also be considered one of its great failings, was the resilience of its protagonists.
Combat in previous editions was fast. Even in 3E, no more than three or four rounds would go by before the big guns had been deployed and the bad guys dispatched. This had its benefits (and DDN is heading back in that direction), but it also reduced many/most "encounters" to dumb shoot-outs where two bags of hit points would simply try to kill each-other using the most expedient methods possible. This was a far cry from the kind of dramatic narrative that ebbs and flows with the fortunes of the combatants and makes for yarns still spun around the camp-fire years later.
In other words, there was no room for story.
I'm not talking about story as in "plot". That's easy. I'm talking about when Luke Skywalker faced down the Emperor. It was an encounter that moved from sly confidence to epic tragedy and unexpected heroism in the space of just a few minutes. I'm talking about when Indiana Jones realised that the Nazi's had gotten away with the Ark, and went looking for a horse. I'm talking about when Neo decided to stand his ground against Agent Smith in the subway station. Long, action-packed "encounters" in the D&D sense, where events are given the breathing room to take surprising turns of direction.
In 3E, more often than not, the climactic encounter -- indeed, any encounter -- would last just long enough for, let's say, the lich to kill one or two of the clothies before the Mass Hasted warriors grappled it to death. Suspense, tension, pacing, heroism...where were they to be found, exactly? Nowhere. The only drama is the urgent hunt for Armour Class and Saving Throw bonuses and a way to close the distance quickly before you get killed. By the book, the poor DM has no tools with which to do anything else.
In 4th Edition, by contrast, the epic was always just a dice-throw away. This was both its power and its flaw.
Online tools - This is a controversial one because part-way through the life-cycle of the game, the standalone Character Builder inexplicably gave way to an incomplete online version of the tool which took two years or more to come up to scratch. Both apps were great in their finished forms, and I'm not going to quibble about that here. Instead I think 4E deserves credit for committing to a rich set of tools in the first place.
In particular, the Compendium was awesome from the outset. An indexed and searchable database of every power, skill, monster, and NPC from the source-books and Dungeon and Dragon magazines. This was second-to-none for adventure and encounter building, and a constant source of inspiration.
The Monster Builder was similarly amazing, allowing me to re-skin and modify any monster in the game with just a few clicks. It wasn't perfect, but in a game where prep time was already short compared to 3E, the Monster Builder made sure that all that was left were the fun bits.
The Bad - 4E isn't perfect, and I certainly wouldn't be as interested in DDN if it was.
Combat length - My single biggest gripe with the system, and in many ways the trade-off for creating a framework that gave encounters the breathing room they needed to really shine, was combat length. It was something I fought with every step of the way.
As a DM you have always had the job of keeping the game interesting. In fact it might be your primary job, and has been true across all editions. Unfortunately what 4E did was give inexperienced (or downright bad) DM's everything they needed to create interminable combats where the PC's were never going to be harmed and the monsters had so many hit points that killing them could take hours.
This was a result of both the encounter framework, some exceptionally bad (or more likely rushed) monster design from the early Monster Manuals, and the decision to call the toughest monsters "Solo's" when they should never be fought that way. Yes, they could hold their own against a full party. No, it would not be a great way to spend a Sunday afternoon.
Having said that, every group is different and my own
players never really thought that long combats were a problem. As the DM, though, it was frustrating to
have entire sessions consumed by encounters that I never intended for
anything other than flavour, or as minor speed-bumps intended to keep the danger level up.
And that was perhaps what was missing: a framework for minor encounters that maybe shouldn't tax the party but should
inform them what's coming up or what's going on. Traditional, online
solutions to this problem ("Just halve monster hit-points!") missed the
point, to my mind, of what made 4E combats so great in the first place, and thus were never an
appropriate solution for my group. In fact, I never found a solution
that (a) everyone liked, and (b) didn't just explode PC power into the
stratosphere. My 4E campaign ended with this as an unsolved problem.
Conditions - These were the building blocks for a lot of the strategy you could bring to bear as a player and as a DM but to track, they were a fiddly pain in the ass. Classes could dish out dozens of conditions per combat, many of them non-standard ("anyone who hits this guy gets 6 hit points, but only if he hasn't yet acted, is next to an ally, and had Weetabix for breakfast"), and while these were great on paper, they may as well have sluiced the entire gaming table in tar for the detrimental effect they had on play-speed. There were far too many of the damn things, and there was no good way to keep track of them. We used Alea Tools magnetic markers for the job, but we really only ever tolerated them rather than enjoyed them, and those magnets were both fun and annoying in equal measure.
Stealth - Oh my lord, stealth. The original version of the rules was so bad that they were re-written entirely in errata. Over time the Rogue PC in my group accumulated such an amazing combination of Powers and tricks that, even standing in a room full of mooks with a spotlight trained on him, he was effectively invisible. He needed to be, and everything he did was kosher, but the rules forced us to create a fiction where we assumed he was affecting his enemies minds in some way. It was the only way to justify the absurd situation playing out on the grid.
Look, in a tactical miniatures situation, stealth simply doesn't work. In the battle-mat-free version of combat, such as DDN's, it's fine, you can hand-wave it away, as you're hand-waving many things anyway, but when the guy is right there in his five-foot square and you potentially have to calculate line of sight for a dozen enemies and then worry about what they can communicate to their buddies, it just falls to pieces. The cost/benefit ratio is way off.
In our game, we kind of got there in the end, with deft play from the Rogue in question and an unspoken contract around the table that whenever the rules trampled all over the evidence of our own eyes, we'd just let it lie. But please, please, DDN, keep it unapologetically simple in the next version.