1. Tracking DPS cooldowns To say that our cooldowns (Avenging Wrath, Zealotry, and Guardian of Ancient Kings, in addition to trinkets and the like) are important to our success as damage dealers would be quite the understatement. Ret paladins bring some of the best on-demand burst damage around, all thanks to our stable of DPS-increasing buttons. Being aware of the timers on these buttons, therefore, is a top priority.
Personally, I have always kept my cooldowns on a separate bar and tracked their timers using OmniCC (a great addon itself), but if you're looking for something a little more dedicated, there are a number of good addons out there: TellMeWhen, NeedToKnow, ForteXorcist, etc.
2. Maximizing Inquisition uptime The solitary maintenance buff for our spec, Inquisition can become a bear to keep track of if you're not very good at picking its picture out from a sea of other buffs on your character at any one time. I would go into how vital this buff is to our DPS, but I really don't need to go into the importance of maintaining Inquisition, right? I thought not!
I use a slightly altered version of IceHUD to track Inquisition, though I have used Power Auras Classic and WeakAuras in the past to much success. The much-touted clcret rotation helper has a very good Inquisition module as well.
3. Holy power display The hardest part about playing on the Mists beta right now is not having an easy way of keeping track of holy power, especially with the addition of Boundless Conviction. To my surprise, however, simply right-clicking the stock unit frame showed the option of moving the frame (and attached holy power bar) anywhere you want on the screen. I was quite surprised by this and felt that it was a great idea, and then I was told that particular functionality has been in the game for quite some time now.
Now that I've wiped the egg off my face (well, most of it anyway), I admit that I am still not a fan of Blizzard's stock holy power tracker. Maybe I'm just spoiled with being able to customize everything, but ever since 4.0.1, I have sought a way to display this information so that I would be aware of how much holy power I had at all times. I used Power Auras for a while but have now switched to IceHUD. If you're looking for another alternative, I know quite a few people who love NugComboBar.
4. Raid frame management One of the best parts about being a hybrid class is that every now and then, we can toss out a helping hand (har, har, har). Unfortunately, knowing just the player to help out and in what fashion can be difficult in a large group. Raid and party frames make these decisions much easier by displaying pertinent information such as health, mana, and (in many cases) relevant buffs and debuffs.
I use Grid for both raid and party frames, but again I have used many others in the past. VuhDo has a lot of out-of-the-box functionality that Grid lacks, though I would argue that Grid is much more customizable in the long run. If you're just looking for party frames, X-Perl, Shadowed Unit Frames, and Pitbull are all good choices.
5. Encounter-related data As a science major, I have had a love affair with data for most of my adult life. Even when I don't understand all of it, my philosophy is that it is better to have it and not need it than the other way around. Therefore, it should come as no surprise that I will use addons like damage meters and boss timers until the day I quit playing the game.
These addons have gotten a bad rap in the past, mainly because many players focus on maximizing their own representation on the chart, ignoring encounter mechanics and basic teamwork in the process. This is not how you should use these tools. The ways in which these tools can and should be used could fill another post or two alone, but for our purposes, I'll just say that for the individual, these addons serve to provide crucial feedback on your performance within the game.
My favorite damage meters have been two of the more popular offerings out there right now, Recount and Skada. Both addons provide mountains of information, from damage dealt to healing done, dispels to deaths. Having all of this kind of information at your fingertips can be immensely beneficial to improving your play or working around some troubling mechanic.
As for boss timers, I have pretty regularly stuck with Deadly Boss Mods. I tried out Deus Vox Encounters during Wrath, but although it is a very aesthetically pleasing addon, I found that it was not updated as often as DBM, nor did it seem to display as much information.
Remember, this sampling of addons is just the tip of the iceberg. Experiment, mix and match, and maybe develop an addon of your own!
The Light and How to Swing It teaches you the ins and outs of retribution paladins, from Ret 101 and how to gem, enchant and reforge your retadin, to essential ret pally addons.