Since I started reading these forums, I have seen Ace2 based mods touted as superior to most other mods, but I am not sure exactly why. As the title says, what exactly are the specific advantages of Ace2 mods? Is it only low resource usage, or are they superior in some other specific ways as well?

Sigh here we go again, the answer is simple.

Use what you want, the people who use only Ace2 mods (or claim that they are the best) are just as foolish as the people who only use non-Ace2 mods, base your choice of addons on the features it provide, not the library it uses which is only the concern of a developer.

Thank you, that is the kind of response that, believe it or not, I was looking for. A definitive "Yes they are just better, that is why everyone uses them." or "No, it is simply preference".

First of all the benefit from using Aced addons for end user might be from non existant (might get worse than standalone even) if only 1 addon using a library to some decent amounts when there is multiple addons, but in general it is a positive thing.
On the other hand some addon authors are overusing libraries when it is clearly not needed. (/wave Nymbia - ArenaPointer used to be dependant 500kb of libraries for an addon that only adds 3 numbers to UI, eventually all dependancies got removed). Embedded libraries increasing GUI loading times.
Most people like Aced addons for ability to update it with WAU, however some addon authors never read the SVN manuals and commiting all changes directly to trunk, often breaking the addon.
Common GUI for most addons. This is a question of what you like. I can't stand dropdown interface for such a complex addon like Bigwigs for example, therefore I use DBM.

I wouldn't say there is really that much of a benefit for addon end user really, but for developer Ace gives ability to create addons quickly with less effort

In the end it's all about what suits you best really

I switched to all ace addons simply because the auto updater is so easy to keep all my mods up to date, and it does have a version of 99% of other mods out there, some of them are a little more annoying to get used to (eg closetgnome vs itemrack) but in the end the hastle of manually updating 10 different mods just isn't worth it.

Omen also has the great function of telling you when someone else is running a new version than you are, This is a great feature and id like to see it in more mods.

This is pretty much why I switched. It is a pain to dig through Curse's messy site to see if Bongos has been updated.

WoWAceUpdater sealed the deal for me as well. I've always liked the high level of polish of Ace mods, but when updating your entire library of mods becomes as easy as pressing F12, I don't think I could ever go back to manual downloading/updating.

There are some benefits to users, although they mostly only pop up if you have a lot of Ace addons. The big one, as others have mentioned, is WAU and its friends - but it doesn't make much sense to run WAU to update 1-2 addons.

The big advantages to Ace are mostly for developers, however. Since there are a lot of Ace libraries, it can rapidly speed up the development process. Need to make some timer bars? Already written. Need a simple config gui? Already written. Need a big list of localized spell names? Already written. Need to parse the combat log? Already written. And so on.

Te downside is that if you've got a single addon which is using a lot of Ace libraries, it'll typically be something of a resource hog. The upside is that if you've got several addons which are all using the same Ace libraries, you'll typically see some resource savings (and probably fewer bugs). After all, what's typically going to be better - 5 addons each with their own custom timer bar implementation? Or 5 addons all using one timer bar library?

I actually found this recent addition to the WoWAce Wiki a pretty good explanation of the who's and what's. It's not really written from the point of view of what Ace2 actually is, but it does a good job of explaining the (dis)advantages.

In particular I personally find that some of the mods on there suffer from very, very bad 'feature creep', that is, additions to the basic mod which should really should be included as a separately downloadable plug-in, but others might very well like that. I recently stopped using Pitbull for that reason; even with disabling the parts I didn't care about in the addon configuration screen I found it taking up too much memory.
I've switched over to a version of oUF someone else was updating instead now, downside is that it's only configurable through modifying the LUA, but that's not really much of a barrier for me personally. It means I get all the features I care about for a way smaller performance cost.

On the upside, at least Ace provides a straightforward mechanism for the modularization of AddOns; Cartographer's instance maps and Quartz's buff bars are two examples of functionality I already get from different AddOns so it's nice to have the option to just not load those modules but keep the other, more useful parts of the AddOn.

I also like the option of being able to use Niagara as a centralized location for configuring 90%+ of my AddOns without having to go through all of their individual slash commands or FuBar/Minimap icons.

Memory usage is a useless number, 99% of the time a user isn't going to notice the difference between 10 MB used and 20 MB, or even 30 MB, when people notice a performance issue, it's typically because an addon is using an OnUpdate incorrectly, or registered for an event that wasn't needed, which isn't really related to memory.

People also have a habit of getting paranoid when something changes, minus the obvious "Onyxia deep breaths more" comments, when something changes you pay more attention and may mistake a random occurrence for a real change.

Given how much cladhaire was complaining about having to setup and just deal with the AceSVN in general, thats probably why. WoWInterface has around 2,000+ addons in the database not including the out dated/unmaintened category, where as AceSVN has around 80-100 and I believe around 40-60 authors which is going to be a lot smaller of a scale then a resource site. Not positive on the authors numbers but I'm sure someone will correct me if I'm off.