1. player focus fire increases effectiveness by eliminating an enemy faster meaning less team enemy attacks.
Target selection in general is very important. You generally need to prioritize eliminating the most damage you can in any given time period, which sometimes means spreading attacks out to eliminate glass cannon targets (often minions or artillery) rather than killing the biggest sack of hp the party can manage.
2. Hoarding daily powers and action points is not a good plan. I’ve seen a lot of players have a tendency toward this. Ideally their best use case is on round 1 or 2 after you’ve been buffed or a target debuffed. Many players only pull out the strong stuff when the encounter looks dire - which is very inefficient.
You also want to look for situations where you'll get the most out of a given daily or even AP to make your overall work day as efficient as you can. Dropping an enemy that might have gotten another attack is usually great, but if saving that resource lets you drop two enemies in a later encounter you didn't get the most out of it. Often a guessing game as to whether now is the right time to unload - which leads to that hoarding behavior. If the party as a whole can coordinate their daily usage you'll be stronger as a whole, with something big to pull out in every encounter.
Well, unless you're all playing essentials classes with few or no dailies.
3. Defenders improve defensive efficiency a lot against most solos with a few minions. They also can increase efficiency if the dm disproportionately targets them as many instinctively do, as they tend toward higher ac and more healing surges. Their defensive effectiveness contributions outside that seem fairly minor for most builds - but their damage output isn’t usually that far off from most strikers (twin strike and frost cheese not withstanding)
Depending on the build many defenders can double as quite versatile controllers, at least in the positional sense. Pretty much any build that lets a class perform strongly in a secondary role is worth considering, especially when the whole party coordinates in that regard.
4. Make sure players are staying up with the magic item treadmill. Being a couple of bonuses down can make a huge impact. Speaking of 4e is designed toward players having significant input into what magic items they can acquire. If this isn’t being allowed, it may lead to a much more difficult game.
Yeah, keep those wish lists up to date and grant at least some of the requests - or something that does a similar job even better if such a thing exists. The math's too tight in 4e to shaft players with junk items or no items at all unless you modify your encounter difficulty to reflect that.
The TL;DR here would be that a party should be coordinating their builds and tactics to play off one another, not working in isolation. 4e is built on group synergies, for better or worse.