I haven't got anything useful to say about durations - I just use them buy-the-book - but I thought I'd say something about the "puzzle" aspect of combat encounters.
EVERY monster should present a 'puzzle' in the form of asking the question 'what tactics will defeat this'. The danger with 4e is in simply assembling encounter after encounter of stock monsters, which then simply demand stock tactics to defeat. So, a monster that deviates more from the norm is useful. They also tend to be things that can synergize well with specific terrain and tactical situations.
its variable what you need. A bunch of old time players are going to lob fire at trolls, they know what to do. A bunch of 4th graders playing for the 1st time? They PROBABLY don't know about trolls and fire... So different things will work for these different groups, and to different degrees. For the old timers you may need trolls that eat fire and need cold iron weapons to kill them, but you better be prepared to have a good story for that
I personally think the "fire vs trolls", "no electricity vs shambling mound" etc thing isn't a very interesting puzzle when it is about player ignorance. I can see how in some games you might use overarching rumour or research mechanics so that the issue of ignorance, and hence of being able to solve the puzzle, is itself an element of overall skilled play, but I don't think that 4e lends itself to this very well (whereas, say, Burning Wheel probably would).
The turnover of encounter is pretty high in 4e, and I think the game works best when the session/adventure structure is fluid, being a consequence of what happens in those individual encounters. And so the idea that the PCs (and, by implication, the players) would take steps now in relation to some encounter an indeterminate time down the track just doesn't seem that workable to me. (Unless its about Orcus, or Lolth, or some end-game type opponent that everyone can see coming - one reason that this works better in BW is that BW tends towards having only these endgame-style encounters.)
In my most recent 4e session the PCs were fighting (among other foes) an 8-headed Primordial Heroslaying Hydra. When a head got cut off, and two more grew in its place, they wanted to know how to stop that happening. So an Arcana check was made, and the Sage of Ages recognised the hydra as one that is vulnrable to cold and necrotic. And so the puzzle aspect wasn't "What do we have to do to stop it sprouting heads?", but rather "How can we deliver cold or necrotic damage to this hydra?" I think that works better in the 4e context, because it is a tactical puzzle that the game is well-adapted to making an interesting element of the game (eg maybe only one PC has cold damage, and is doing stuff over there, and we somehow need to get that character engaged with the hydra over here without allowing the situation over there to escalate out of control).
I haven't really found "stock tactics" too much of an issue in my 4e GMing. Of course there are some repeated patterns - each player's PC gives rise to some particular suite of resources, and so of course the fighter wants to get in the middle to unleash Come and Get It or some other close burst, and the sorcerer wants enemies grouped (but without a fighter in the middle) so they can be targetted with Blazing Starfall, etc.
But with a reasonable mix of NPCs and monsters, and some decent terrain, it's not that hard to force the players to mix it up a bit and think on their feet. Eg in the same encounter with the hydra I had acid rain falling for 10 damage at the end of the turn, with shelter available in a temple but that means getting into the aura of the Godforged Colossus and/or into the burst radius of the Soul Gem. (The fictional situation is one of the PCs being in Carceri trying to hunt down Miska the Wolf-Spider.) The temple was on the thinnest part of an isthmus, and the PC ranger got knocked over the edge into the acid lake.
This sort of thing keeps the players on their toes, and 4e - with its good variety of monster special abilities and its relatively reliable and robust mechanics for resolving forced movement and other effects - makes it pretty easy to implement.
On a recent thread on the 5e board someone was criticising the idea of "balanced encounters" on the grounds that 5 orcs in a 10' by 10' room and 5 balors in a 50' by 50' room is ultimately much the same, and hence boring or at least unmemorable. The claim is probably true, but I can't verify it from experience because there's just no need to run those sorts of encounters in 4e. This is even the case at 1st level: my first ever 4e combat encounter was adapted from Night's Dark Terror, and involved the PCs' boat being stopped by a chain across a river. There was a sandbar or two, enemies on a raft, the current rules from the DMG, and PCs falling into the river after failing Athletics check but then eventually gaining control of the enemy raft after (if I'm remembering properly all these years later) the warlock teleported onto it. And in our last Dark Sun session - still at 1st level - the PCs fought some templars in a dry gully with DT on its silty floor, fairly steep sides that needed an Athletics check to climb out of to escape the melee but also allowed taking cover by lying prone, and some buildings near its edges for taking cover. Both PCs and NPCs were moving around in that encounter, trying to bring forces to bear and establish or maintain flanks, and trying to get out when the damage was stacking up too badly.
You don't even need especially imaginative monsters to do this sort of thing. (In both the 1st level encounters I've mentioned, I don't think the NPCs had anything much more exciting than melee and ranged basic attacks. Maybe one special "leader"-type in each encounter.)
