A few addons are way more complex than others but here is a list of addons I use for both my healers and dps classes that I find important. Main thing is to get usedto them days before your raid so you aren't freaking out at wtf ever is ringing/flashing/positioned in the middle of your screen.
~~Pitbull 4.0 (only for myself/target/target's target)- fairly complex compared to some, easier versions are Xperl, and many others (no experience with other than what I named). These tend to eat a lot of memory.
~~Grid- unit frames
~~Grid Raid Debuffs (VERY handy- places icon or however you choose on raid members frames denoting what debuffs they have during specific encounters- think Jarraxus's Incinerate Flesh for example)
~~A cast bar addon to centralize your cast bar (I use gnosis for spriest and EZcastbar for other classes)
~~An Event alerter- for showing procs, gains, etc. A lot of people swear by Power Auras, which is very very complex. Easier solutions would be Event Alert, Need to Know, etc.
~~A debuff/buff watch- To track things like PowerWord:Shield, Weakened Soul, Grace, Inspiration. ClassTimers and Quartz are fairly simple to set up.
There are tons of threads of people's UIs on this site- check them out and do what makes you most comfortable. I am a fan of raid frams being directly under my character with my health bar and my targets health bars on each side of me.
I'm not a fan of many macros on my priest, I think EJ has a list of some, also maybe check out the blog Divine Aegis and other priest blogs linked on that site.
Best of luck, hope this helps a little!
im pretty classic i guess, i use grid, with pretty all the extra components (mana bar, etc). and power auras to anounce me when my surge of light procs. i dont have any macros for healing. i have a mouse-over but it tends not to always works. i press my skills 1-2-3-4 and shift1/2/3/4. pretty simple and effective..