Haha, I'm the only one to vote for the edition wars? (I also voted for skill challenges and paragon paths.) What can I say? I think the conversations they sparked are actually really valuable when it comes to evaluating game mechanics (and why I might value certain mechanics over others). I'm not much into it anymore, but I think that's because those conversations really helped flesh out my views on gaming, and let me verbalize what I like in a way I hadn't been able to prior to many of those conversations.
Communication is useful, even if it's destructive.
Mechanically I'd say that the best thing from 4e was completely decoupling hit point recovery from spell-casting. Just that innovation was enough to really break open the wall. This introduced options for self recovery.
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The second one in my book is transparent mechanics, which ties directly to consistent maths.
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The third would be self-contained monsters. Easy to use, easy to wing, and easy to prepare. A delight to be used in combat and a delight for preparation time.
I'd also add a ditto to everything [MENTION=6696971]Manbearcat[/MENTION] put on his post.
I forgot to add characters that play as the desired concept from the beginning rather than waiting 3, 4, 7, 11 levels before becoming the character concept the player envisioned.
My main love is the explicitness. The designers were explicit about the maths so that player groups can use it and morph it if they must to create the game they want. Power effects are explicit so that the game does not become an enforced "beauty contest" where, if the GM likes* your idea it will succeed (albeit you might need to make a roll), but if they dislike* your idea it will likely fail ("well, you need an athletics check to catch up to where the giant is, followed by acrobatics to jump on his back and a STR and a DEX check to stay on. Then roll your attack - but you get a -4 modifier because he's thrashing about").
This is another "other" I agree with (but didn't think of when I originally posted). I would widen it, though - the whole minion - standard - elite - solo - swarm* spectrum allows combat encounters using monsters from the full range of "levels", but scaled and adjusted to suit the current PC level well. The only thing really missing was a stage between minion and standard - which is why several house-rule variants of such a thing exist.I voted Other. Minions were the big innovation that I really liked from 4E. I think the idea was great and really lends itself to creating cinematic, dramatic combat scenarios. I'm not a 4E fan, but the Minions rules are inspiring.