
Disable all your add-ons, take a screen shot. That's about what I am aiming for. No really, one of the goals of my UI is to not do anything which would cause a learning curve should I be without some or even all of my add-ons. In fact this just recently happened to me on my laptop which I updated to 4.x on the road and just wanted to play my warlock a little without messing with add-ons. I also had an ulterior motive to see how the default UI looked, I've not used it in an age.
So my first test was to see just how awesome the new quest tracking system was. I've been using Carbonite since pretty much forever and much to my surprise the default UI is actually quite good. Carbonite has only two perks: more flexible placement of the quest tracking map and more automated quest tracking by proximity. I can almost function with the small map up there in the upper left where blizzard stick all thier informational frames, but I really like the translucent map Carbonite provides shoved off in a more discrete corner. The other thing is that Cabonite automagically tracks quests based on your proximity to them, with the default UI you have the option to track every quest you pick up, or do it all manually. The tracking selection isn't a big deal; if the small map frame was more flexible in placement that would be enough to get me to drop Carbonite.
Once I got into questing after a brief self tutorial I noticed the one place the default UI really fails, inventory management. I think this has more to do with the play style I've become accustomed to than blizzard's shortcomings. I use ArkInventory to show my entire inventory in one frame sorted by item type. I know exactly where my quest items are going to be, I know exactly where my BoE greens and enchanting mats are when it comes to send they to my enchanter, etc. Locating quest items used to be a big to do before the items were available right on the quest tracker, but occasionally they are missing and you have to quick open your bag and use them, often in a bit of a hurry since if you are like me you didn't look for the item before you had to use it right before a mob dies. I don't want to organize my bag and I certainly don't want to hunt through it. The only time I look in my bag for the most part is to empty it or sometimes equip a new item. Even then I have an add on to throw away the cheapest junk item I am carrying to make room without ever opening my bag. Pretty sure inventory management add-ons are here to stay, there is no way blizzard will add the level of tracking most people come to expect, including item counts on alts and guild banks, searches, etc. It is just too far beyond the scope of the default UI.
The next biggie for me is equipment sets, pretty much all my toons have at least a PVE and a PVP set. My hybrid toons have more. Right now my druid walks around with 4 complete sets and a few pieces of frost resist gear for special occasions. Without an equipment manager I would be totally lost putting on a specific set manually. Luckily not too long ago this feature was included in the default UI and better yet the add-on I used previously, Outfitter, makes use of the server side equipment sets so with or without the add-on I am good to go. The reason I still keep the add-on around is for partial equipment sets, enhanced tootips to let me know which sets and item is in, and the oh so convenient list of BoP items not currently in a set; that is such a great way to sell/equip stuff quickly.
So far I've not touched the default UI at all. In fact most of my addons are hidden behind the scenes unless activated. The most notable element of my UI is my DataBroker bar across the top of my screen. Here I have access to a lot of otherwise hidden add-ons as well as useful information at a glance. How much gold I have, what equipment set I am using, my equipment durability and anything else I want to know without clicking somewhere. I actually don't use most of the space so I am considering moving to a DataBroker add-on that lets me put things all over instead of on a bar.
Given my large widescreen monitor and my UI scale set as low as possible, I have a lot of room at the bottom of my screen to the left and right of my action bars/utility buttons. On the left I have my chat frame which you can't drag all the way to the bottom by default so I am thinking underneath is the perfect place to hide that DataBroker stuff I mentioned. On the right I have Omen and Recount just below my tooltip frame and additional action bars on the right side. I always have all the action bars enabled and share a common organizational system between toons.
The only other noticeable difference between my UI and the default (out of combat) then are my Pitbull unit frames. I am currently reevaluating whether or not the default unit frames are sufficient for me. With the new enhancements I think they might be. All I really use the unit/raid frames for are highlighting debuffs, aggro and the incoming heals display. I have some minor concerns about the numbers displayed, but that alone wouldn't be enough reason for me to keep an entire add-on.
In combat I have a few additional things pop up in the middle of my screen. I track combo points and rotation specific buffs and debuffs front and center with NugComboBars and Auracle. I was actually half thinking about power aura, but then blizzard added a simple version which works quite well for me. Along the same lines I replaced the default cast bars with Quartz. The main reason is basically just for the latency display which is invaluable when chain casting. I can start casting the next spell before I actually see the first go off.
I suppose WIM (WoW Instant Messenger) is also worth a mention. I don't know how often people have the need to keep multiple whisper conversations straight, but I do so with some regularity. Having a little window for each conversion is quite handy and I would be a lot less chatty without them.
So there you have a basic idea of what my UI looks like and have a point of reference should you ever find your UI or add-ons the target of my mockery. For example a bar mod screwing up a boss encounter will probably feel my ire for a good six months. Lets face it, Warcraft is buggy enough without introducing more potential points of failure, especially around major patches such as these. I like non intrusive add-ons which improve my gameplay, but don't take over. A little extra tooltip text, another button or informational display in a relevant location, those are all nice things to have. Like knowing who is responsible for a buff by mousing over it. A minor detail, but I seem to be the only one in raids that knows. :)