I have learned many things from working on social games, and one of them is that you should show your players the countdown timers.
Why am I talking about this? I started playing Final Fantasy XIV, and they have a lot of countdown timers... and many of the are hidden. In this MMO, the most efficient exp is earned from levequests, a special kind of quest. You can do eight battle leves per day, and eight crafting leves per day. Or, well, maybe it's every 36 hours? I don't know for sure, and most of the players don't seem to know either. This is a critical cooldown timer, and the game doesn't show it to you! It's extremely frustrating, because you have to go to a certain NPC in town to exchange used leves for new ones, but you can't exchange a used one until 24 (or 36) hours after you completed it. Without the timers, you just have no idea when to go back for them.
That's not the only important cooldown timer the game doesn't show you. After you die and revive, you have a weakness status. In the previous Final Fantasy MMO this lasted 5 minutes, and now it's certainly shorter, perhaps 2 minutes? I haven't timed it yet, but I shouldn't have to. The timer should be displayed for you so you can watch it tick down. All of the status effects do a little bit of blinking before they wear off, but some of them seem to blink for an awfully long time - maybe they blink at 50% maybe 25%? Again, I have no idea, and it's maddening.
If your game has cooldown timers, show them. In FFXIV, all the abilities you can use show their cooldown timers in minutes and seconds, ticking away. Why couldn't they extend that to everything else?
There are many ways of showing a cooldown in a way that a player can feel they have a good grip on how long it will be until... whatever it is happens. Minutes and seconds is the most accurate, but it can be a little ugly and possibly confusing if you load the screen with dozens of them. I also like the radar-sweep transition from greyed out to normal (or vice versa). It's almost as precise as numbers, and, as long as there is enough of a contrast bteween the "live" and "dead" shading that's progressing around the icon, is as easy to read.
In social games, cooldown timers are even more important, because they are often very long - hours or even days. Players often leave and come back to the game later. They need to know how long they can be away, and when they do return, have to be able to easily reacquire the amount of time remaining on multiple timers. In these games numbers are usually the only acceptable readout.
Of course, you can always do both a radar sweep and numbers, and I would welcome it. It's a lot better than nothing.