These are a few thoughts on 4th edition D&D after running a 2 year campaign...
Preamble: So I started playing D&D with the Basic set, the old red box with the insanely cool picture of a fighter slashing at a Red Dragon. Man that brings back memories! My first game was an adventure set in B4, my axe wielding Dwarf slew two fire beetles before dying to a trap. Played AD&D (1st) at highschool, many adventures, good times. Long hiatus, missed 2nd edition totally, played a short 3rd ed adventure in 2001, and a longer 3.5 ed game in 2003/4. Another hiatus and then played some more 3.5 ed 2006/7, enough to get a feel for it. When 4th edition came out I picked up the PHB and eventually got a group together for 4th edition. We have just finished session 34 (we aim to play twice a month), the characters are level 12 and currently in the city of Halfpeak, Shadowfell, deciding whether to support the Cult of Pleasure, or the Velvet league or set themselves up as an alternative power structure. I have 6 players of which we usually get 4 turning up on any given session.
What is different? As a DM, 4th edition is easy to write up encounters, monsters are much easer to invent it takes 10-15 minutes to come up with some suitable monsters with interesting powers and some sort of terrain setting. I can remember monsters being much harder to write in 3.5 edition. Terrain is very important though, the old days of bash the door in and trash the occupants scenario is gone. Combat is fluid with lots of movement and terrain plays a big part. Having encounters that flow together is a good way of increasing threat levels and rewarding smartplay.
Number of players. 4 is perfect. 3 or 5 works, 5 can be a little slow in combats and 3 you have to be careful if a character goes down the party is in trouble. I usually add one of the player's characters who is not present so that we always have 4 characters in game and encounters are set for 4 PCs. I have run sessions for 2 players and provided you tailor it, it works. Add some healing potions if the leader is absent or use a lot more minions.
Combat roles There is a lot of discussion about combat roles, some people hate the idea, some love it. From my side of the screen I can say it makes little difference as you can tailor the party or the monster's strategies. As long as you have some kind of leader (a healer) and something to intercept monster melee types (a defender, summoned creatures, or those annoying wizard zones etc) you are fine. At one stage my PCs had 4 strikers and a leader. However, overall I would agree that the party has worked best with a defender, leader and a number of strikers (melee/ranged). Controllers are the weakest "role" type and hardest for my PCs to grasp, I can see if they were played well they would make some encounters total cakewalks. Every wizard we have seen has wanted to play a blaster striker type, which is fine per se but they seem if anyting underpowered against other strikers. The party currently is a shadar kai swordmage, halfling rogue, deva shaman, deva sorcerer, half elf warlock and an eladrin wizard. The rogue does the most damage in this party but also spends a lot of time taking dirt naps, the shaman is my most annoying PC as the summoned panther can really irritate my monster plans.
Combat can be awesome. One of the strengths of 4th ed are the encounters, done right they are kickass fun, done poorly they can be forgettable affairs. Definitely a plus of the system.
Character deaths I roll dice in the open. In 3.5 and 1st I used to fudge my rolls to keep the game going, to date there has been no need for this in 4th. The system is well structured, in 34 sessions we have had 3 character deaths. One was a wizard who was jumped by an elite assasin solo, the party could not come up with a coherent plan and the poor arcanist was picked off as the party kept deserting him to chase lures. The second came from a roleplay event where the character, a barbarian, sacrificed himself to allow the party to escape. The third was a total surprise, an even level ranged ambush on a glacier, the monsters were intelligent and recognised the cleric who was healing the party, they then focus fired and the due to ice and 3 bad saving throws the cleric expired. It is hard to kill characters in this edition! My players know this and are sometimes quite rash. It leads to exciting tense fights.
Combat got grindy. The monsters in Monster Manuals 1 and 2 had either too high defences or too many hitpoints leading to long boring fights where the adventurers had exhausted their powers and were reduced to at-will attacks. This problem was not apparent in low heroic but was especially noticeable in late heroic (levels 8-10). We had one combat encounter last 2.5 hours and after that I spent a lot of time trying to speed up my combats. Late heroic characters seem to be behind the curve on hitting and damage. Using MM3 and Monster Vault (probably the best product of the line IMHO) has helped a lot, I also try to avoid monsters with 15 different condition abilities, 1 or 2 is enough to keep track of. Minions are also a great way to stimulate combats.
Minions - a great invention. After low levels they are best used in waves, especially once the party is comitted. Ranged minions really annoy my PCs and the plink, plink can certainly add up if they are not addressed. A big plus of 4th edition.
Skill challenges mixed bag. Out of several skill challenges only 2 or 3 have really gone over well. I do like to use them in conjunction with combats (close the portal, stop the reanimators, get rid of a trap etc). Good old roleplaying is much more fun in social settings.
XP is totally irrelevant with the way 4th works you can simply abolish XP and have the characters gain levels as and when they achieve their tasks and milestones. My PCs gain a level roughly every 3 sessions, a session usually includes 2-4 "encounters" which often involve combat and/or roleplaying/skill challenges.
Curses/diseases are a great way to stop the 15 minute adventuring day. This gets aroudn the whole daily power issue. Daily powers are very swingy, if the PCs go nova and use a lot of daily powers they can win fights quickly. This then leads to them wanting to recharge by taking an extended rest. Using story and various disease/curses helps to avoid this!
So all up, we are having a blast with 4th edition. It does not inhibit roleplaying, it has certainly added a lot to encounters and is working for our campaign. That said I have found WoTC adventure material to be quite disappointing compared with the previous editions (think of Expedition to Barrier Peaks or Saltmarsh Series) and/or Paizo. I have used no WoTC produced modules and ripped off very few encounters either. The whole Essentials "debate" has largely passed us by, no one is using Essentials characters. I can heartily recommend the Monster Vault, (and MM3 is quite good). There are almost no rules debates, either which is a nice change from my old 1st edition days!