So I know a lot of people have their favorite systems and that there is quite a bit of worry that WotC will not deliver a product true to the D&D line with 5e especially after what many people consider a disappointment with the changes they made in 4e. I've been messing around with the playtest and trying to stay up to date on a lot of the discussion across various sites while doing my best to approach the new edition with an open mind.
Lets start by taking a look at previous systems and what about them could have used improvement, and then later see how Next approaches a lot of those areas. And let's keep the discussion mature and constructive and not simply dissolve into edition wars.
Basic D&D
The game that started it all. At the end of the day though the basic game had very little variety and a lot of things boiled down to chance or randomness. The game was designed to send players off to explore some dungeons and didn't focus much on what players might want to do before or after the dungeon, or that they might not decide to go to a dungeon at all. This edition spawned the whole genre, a slew of other games and content from other publishers, and numerous video games and CRPGS, but it could strive to be so much more.
Advanced D&D
AD&D moved the game forward and dumped a whole new mess of races, classes, spells, a more sound combat system, and more mechanics in general. But of course with more content comes more complexity and it wasnt destined to last long. Combat became needlessly time consuming and everything now took too many steps making things hard to imagine, spell casting multiple steps and by the time you were done things could have already changed, and most people felt THAC0 needed to be removed. Outside of that, the designers tried too hard to make the game more realistic and while it failed in some places in others it worked to well causing issues like innate sexism in things like ability scores. Overall the system was mostly just too complicated so that most people never effectively understood how to play and reference books were frequently needed to be flipped through.
2nd Edition
Second edition brought roleplaying forward to become a much more prominent aspect of the game. It also allowed for much more customization of your character and the basis of the skill points system was starting to take shape. However many of the vices from the previous rule sets still plagued it: THAC0 was still and issue and made the game complicated and mathy even for intermediate players and while the class system was starting to take shape as well, there just wasn't enough content to allow characters to become mechanically different than any other member of their class.
3rd Edition / 3.5 Revision
This section could easily be larger than every other section combined, but I'll try not to ramble. And before you burn me at the stake for heresy though know that 3.5 is my favorite system. That said, its not without its own flaws, some of them glaring. When WotC bought the rights to D&D they went about trying to right rules to iron out any and every uncertainty that a player or DM might come across but in some regards they just didnt know when to stop. There are a lot of good things about this edition; The class list finally rounded out to include all the 'core' classes we think of in RPGs today, skill checks swapped from being percentile based to using the d20 system that was used for attacks and saves, with the picking from tons of feats or slotting your skill points allowed you to make unique characters even at level one, and with the various classes and prestigue classes one could hone their character to be much more niche than the standard base classes. The problem though, is that while the core system became less complicated and more streamlined with the d20 system turning the game into "roll a d20, add a number, see if your number is higher than another number", the rules seemed to want to capture every single aspect you could ever encounter and then slap a ruling to it, rather than DMs being able to have some wiggle room when it came to some things now every action normally involved flipping through a pile of books to find the exact ruling for that situation because there was an official one somewhere. Too many specific rules, too many keywords, too many dead levels, and just a ton of math in general bogged things down even though the system allowed for so much creativity. Declaring an attack was easy, making sure that all your math was correct, especially at higher levels, could often become a nightmare, especially if you were doing anything outside simple hitting something with a sword. The game was unnecessarily large and the more that was added the more than it became unbalanced. 3.5 was created to help smooth the balance issues but a lot of the issues above still remained.
4th Edition
This edition often receives a lot of flak for basically not being 3.5, it often gets tagged with being called "WoW on paper" or "Not True D&D" but a change to the gaming license by WotC also ensured that this edition just didn't receive the followership from both players and publishers as it could have had. WotC saw what a convoluted amount of math 3.5 had become and decided to steer away from that and they basically succeeded but in doing so they also seemed to lose a bit of the roleplaying crowd along the way. It was a simple edition that was easy to get into, easy to understand, and powers could be printed out on cards so that even the newest of players could look down and for the most part know what they were capable of. But the game became more or less became solely about combat; the skill system remained but much like WoW's it became neutered to a point of simply getting trained in a few skills at character creation and then leaving it at that, even spells that shouldn't rely on combat now often required combat to work as seen in numerous healing spells that required you first hit an enemy, and most classes were broken down into a few simply little archtypes that spelled out exactly what your job was and what you'd be doing for the life of that character. Though the system fixed the inflated math of 3.5 it essentially meant that every few levels you'd gain one point to attack but enemies would gain a point to armor class. At levels where players would be getting new magic gear, new ability points, and new bonuses... enemies likewise increased in attacks and defenses at the same rate. And while this meant a sort of balance, it also meant that the game was more or less the same at each teir; your chance to hit successfully never really went up, and while your damage increased, so to did the monsters HP which mean that the percentage roughly remained the same.
<Breath>
The skill system looks nice on paper but once you start taking scaling DC's into account things become if more troublesome as the scaling attack/defense conversation above. The way DCs scaled up as you leveled you were never more likely to succeed at a task than you were at level one. And thats assuming that you're trained, if you weren't trained in a particular skill, since they were only based off your ability score as you leveled you actually became less and less likely to succeed at a task. There were feats and options out that that allowed you to put a dent in this problem by increasing your non combat prowess or letting you train more skills and such, but due to the whole system centering on combat taking this was essentially shooting yourself in the foot when it came time to face a solo monster or a tough encounter. Everyone else would be combat centered and min/maxing, and if you didnt you quickly became an encumbrance to the party.