I think essentials being a bit more unpopular is going to make getting content for it hard. But I appreciate the advice so far. There are things I like from previous books but I am afraid of opening the flood gates of complication and imbalance by taking them.
DnD just has a history of making supplements that serve no other purpose than to unbalance the perfect game they made just a year before.
1st- unearthed arcana
2nd- skills and powers
3rd- phb2 and the random complete books they made after they already made a complete book for each class
I just am worried if I add to much everything will tilt
A couple of things (disclaimer, purely in my opinion) about essentials and the current state of the art in published D&D.
You have been away and your memories where from a time when WotC was publishing too much, too fast with too little quality control.
You also then missed the massive correction that happened when errata started coming hard and heavy.
You need to let go of the, "this is what happened when I first played 4e" and look at the lay of the land today.
1) WotC is now publishing less material with much higher quality, which in turn has required less "fixing" in errata.
2) Essentials classes have, for the most part, been pretty well balanced and work well with the previous AEDU classes. In play they act just like their counter parts, the main difference is presentation.
3) Powers are pretty well balanced (with a few notable exceptions, yes Twin Strike I am looking at you) its the feats you have to examine a little closer. And by examine closer I mean allowing feats from pre-essentials material to mix with essentials.
4) The other area that still hasn't been addressed is mundane weapons from Adventurer's Vault (Full Blade, Executioner Axes, etc).
In all, I would allow anything published in Heroes of the [Adjective] [Noun] books, Heroes of the Feywild/Shadow, Neverwinter Campaign Guide, Mordy's Magnificent Emporium. Most Dragon stuff from the last year is good (provided you use any errata out for it). Stuff prior to a year ago I would examine closer.
For all the above, my main criteria would be, does it fit the campaign I am running? If it thematically doesn't fit, then don't use it.
If you are going to use material prior to the publication of essentials then I would allow the errated races (sight unseen), most powers (with errata), and be very selective about feats.
Magic items are a whole 'nother issue. This is another area where some poor design choice prior to essentials compromises the whole-sale use of material. I start very conservatively and only allowing stuff from older sources after vetting. With Mordy's book there is now enough stuff to hit a lot of the basics using just essentials.
If you aim is super low overhead as a DM, then I would restrict to just essentials sources to begin with and slowly expand out of it. If you want a little more variety, add all races and some select classes (say the Dragon write ups of the PH1 classes like the Warlord, Cleric, etc.). If you want even more variety then open up all non-essentials classes. I would keep a firm hand on feats and magic not matter what. I also would ban all weapons from AV1.
One last thing. This should be a negotiation between you and the players. Yes, players will want it all. But if you blanket ban stuff without input from your players you miss out on making them part of the vetting process. You just have to be ultra clear what criteria will allow something into the campaign.
Also, make sure you use monsters that have had the MM3 math changes. It will keep things from getting too grindy (though part of that is encounter design). There are several threads on keeping combats from devolving into spamming low damage at wills vs. opponents with lots of hit points left.
My two coppers,
