I feel like 4E was hardmode on GMs. Every combat encounter is a setpiece. To work well the terrain always has something interesting, like rickety bridges, or lava flows, or gravity wells or something. All the monsters have differences and special things, controller/defender/lurker etc, AND the objectives are often also complex, e.g. stop the ritual before the rift is complete.
These make for great setpieces, but 4E struggles outside of epic setpieces.
As has been said, the 1/2 HP thing is pretty common and is rather needed at higher levels. I also have most NPCs do extra damage when bloodied, to make up for the action economy issues of enemies dying.
One practical tip is that if a player interrupts to use a reaction, they have to use it. They use it immediately or lose it. No umming and ahhhing over it. If the reaction does not work in that situation or work the way they thought, tough, they lose it for interrupting. Some of my players would stop every single action to tell the table what they 'might' or 'could' do about it.
Why would have encounters which are not interesting plot points? Again, study action movies of all sorts. Yes, there may be scenes which include some incidental action; the hero takes out a couple minion guards in the courtyard, etc. but those really need not be run as encounters. They can simply be narration, or if there's some interesting and challenging aspect to them maybe all/part of a Skill Challenge.
DMG1 says it outright "skip to the good stuff." and several other analogous statements. 4e is not made for running long sequences of 'filler' where you have to battle the orcs in room 27 to get to room 34 and battle more orcs just to get to run 82 where you find 5 more orcs and some terrain, etc. etc. etc. This was the whole problem with KotS, and what made it a crap adventure.
Organization is another thing that is really rewarded in 4e, on the part of the players mostly, but the GM needs to be fairly organized as well. Everyone should have power cards, just do it. Write them by hand, buy them (I think that is impossible now) or find a template and print some up. It really isn't that hard, and 'condition cards' are not a bad idea either. When the game moves along, then everyone stays engaged and play is quite quick. It is when Sue is off in the bathroom and Joe is getting a drink, and Fred does something, and then Greg says "wait, Sue could Interrupt that" etc. etc. etc. Now someone else wanders off while you wait.
I hear you in terms of making people take their decisions quickly and all, but you have to make sure they have the tools to reliably make good ones, otherwise players will become frustrated when they are rushed into triggering something and it was useless and they lost it.
Frankly I think 4e is a very easy game to GM, it does require thinking up action sequences, but there is such a vast repertoire out there in movies to inspire you! Not every one needs to be an epic scene either. You should also be ready to improvise. If stuff bogs down suddenly, then foreshadow the arrival of reinforcements or something, make the players push! I think it is also OK to just kill or surrender a monster where the conclusion is inevitable and nothing more interesting is going to happen. It is rare, but it happens. Thing is, there's a formula for play, there's a VAST array of stat blocks, they are easy to reskin/modify, and you can add traps, terrain powers, etc. to make things even more exciting. It is a very easy game to run on-the-fly with little prep.
Also, use MM3 grade monsters. MM1 and even MM2 monsters are often clunkers. There are many good ones, but it is harder to tell. At least go through them and make sure they will work how you want.
If 4e is 'hard' for anyone, it is the players. OTOH they have a very rich repertoire of 'stuff' to hook onto on their characters to tell a good story. So it may demand them to be a bit on their toes in combat, and want to really go for the gusto, but it rewards them with lots of "aha, THAT is the new power I got because I made that pact!" and whatnot. Look up some of
@pemerton's descriptions of play. He shows how to do some really fun stuff, like extrapolating existing mechanics through page 42 and things like that.