My lessons based in part from KM's list:
* I don't care about the Encounter, I care about the Adventure: I think I'm missing some nuance here. I see Adventures consisting of many encounters. What am I missing?
* I want to do more than fight things. 5e should develop non-combat activities but any sub-systems should be optional rules IMO based on some unified mechanic. That mechanic is Skill Challenges, although improvements are needed.
5e should have a separate track for non-combat character progress. Feats should be in separate combat and non-combat pools and each class should have the same # of feats from each pool.
* Combat: 4e combat is 90% fine, but work on streamlining it. I'd focus on conditions (like save ends, ends on my turn, your turn, etc. - too complicated) and incorporate (perhaps as optional) rules like doing max damage vs bloodied target.
I use 4e combat for set pieces (like "zooming in" at a tactical level). I use Skill Challenges for more narrative-based combats (a strategic level). But a "middle-road" should be developed here. Right now, I've experimented with modified Skill Challenges, but it hasn't hit the mark IMO.
* Powers: I like them. I like that different classes can use them (whatever they may be called). Make power sources more flavorful though, but as optional rules. Think Dark Sun - arcane powers are much more flavorful there.
Powers should be balanced. One reason 4e has saved me time (as GM) is that I didn't feel the need to scrutinize each and every PC sheet for a "broken" power that would destroy my campaign. Give us (optional) measuring tools for these powers to modify them and be able to gauge their level of effectiveness. IOW, I don't mind breaking balance so long as I know what I'm getting in to. But it's easier to start with balance and break it than the other way around.
* Rituals: Like the idea of them. Like that GM has control over these potential campaign-breaking powers. Just improve the implementation (specifically, the cost). When I say "cost", I mean the mechanic that governs how often they are used. I'd prefer a mechanic other than gold.
* Roles: I like the idea of roles; keep them but develop them for non-combat as well.
* Treasure: I do agree that 4e treasures can be pretty lame. So, spice things up. I prefer less treasure but more powerful items (that can develop over time), but I can see others may want their christmas trees. So, offer options and optional rules.
* Monsters: 4e did it right with monsters IMHO. Definitely offer electronic support (Monster Builder). I like the single stat block that avoids separate referencing. But create a way (like a point system) to create monsters and their abilities. This way, we can have greater variation, and if they're "not balanced", we'd know (and use them anyway)
. I do this now in 4e but it can be hit or miss.
* GM's burden: 4e has made GM prep very easy IME. 5e should keep that and make it better (easier). A key to this, of course, is the Monster Builder. But make it better - like making Terrain/Hazard/Trap Builder(s).
* Stunts: Refine and expand page 42. Encourage and teach players to make stuff up.