I've been installing and recommending Linux Mint (main edition with Gnome 2) since version 9 LTS to many people who were new to Linux. Linux Mint with Gnome 2 has been hands down the best "just works" distro that I'm not afraid to give to newbies. After the arrival of the unholy mess called Gnome 3, I've been desperately looking for an alternative "just works" system I could give to people, and this time to my wife, too.
I've tried quite a few options and here are the results:
KDE: I had the highest hopes for this. It's very feature rich, very configurable, has some really nice applications, but unfortunately has the occasional major glitch that makes it less than ideal as a "just works" system. I wish there was more focus on stability and QA versus new features. It's also the heaviest, which matters even on new hardware: Battery life suffers on laptops, and the system is just not as snappy as the alternatives. Dolphin opens quite fast but noticeably slower than other similar file managers, it's also slower to browse e.g. Samba shares. It's the most user-friendly graphical file manager with the best features I've ever seen on any OS... but you trade snappiness you could use 100% of the time for features you'd find useful maybe 5-10% of the time. Kickoff is very good, it's the second best main menu after Mint Menu in my opinion. The default visual appearance of KDE is too flashy and busy, but luckily it can be toned down.
Verdict: KDE could be used as a "just works" system, it's quite good but not ideal.
Gnome 3: I imagine it might work for some people who're very new to computers, are very light users, or just have an extremely different idea of what a workflow should be like. For very many people who are used to the traditional desktop, it'd be a show-stopper because it imposes its weird ways on you and cannot be configured differently, so as a "just works" system it's completely useless. Another issue is that it doesn't work without 3D acceleration, so if there's any problem with the 3D drivers (not unusual on Linux), the user would be forced to use a totally different interface. That would be a major annoyance at the very least.
Verdict: Not a viable option. (And I couldn't support it as there's no way I'd use it myself even temporarily.)
Unity: Regarding the user interface experience, it's like Gnome 3, except not as bad. Luckily it works in 2D, but it has a different drawback: It's only available in Ubuntu and derivatives. So it's very hard to support unless you use Ubuntu yourself, and it's also a sort of vendor lock-in: If you get someone to get used to Unity, they'd have a hard time switching to a non-Ubuntu distro.
Verdict: Not a viable option unless you're totally committed to Ubuntu (or derivatives). Might work if you are commited, but isn't ideal because of the unusual workflow. (I couldn't support it as there's no way I'd use Unity myself even temporarily.)
XFCE: Light and snappy but lacks a few convenience features and could use some more configurability. Hopefully it'll get better in the future.
Verdict: Might work in an otherwise very user-friendly distro like Mint but isn't ideal as a "just works" system for newbies.
LXDE: Like XFCE except even lighter and even less convenient.
Verdict: Might work as a last resort but definitely not ideal as a "just works" system.
MATE: Looks very good in the latest LMDE. Snappy enough on modern hardware, not as many features and not as configurable as KDE but it's good enough for the average user. Haven't seen any major glitches with the latest version, but if there are some, I expect MATE to reach the level of reliability Gnome 2 had relatively quickly. The Mint Menu is the best of its kind on any OS in my opinion, a very welcome addition for the average user.
Verdict: Best option as a "just works", "no surprises" system, the only downside being that MATE isn't yet as mature as Gnome 2.
Cinnamon: First of all, kudos to the Mint team for taming Gnome 3. That said, Cinnamon is not yet polished enough, not even close to being configurable enough, and doesn't work without 3D, so as with Gnome 3, if the 3D drivers go down, your inexperienced user will have a very hard time with a totally different interface. Currently there's nothing that Cinnamon does better than MATE from a user interface perspective but it's less polished and more fragile.
Verdict: Might work but it isn't a good option. (And supporting it would be annoying as I really don't like it myself.)
Overall ranking: MATE > KDE > XFCE >> LXDE, Cinnamon >>> Unity >> Gnome 3
Since I'm an advanced user who mostly lives in the terminal and can fix most problems, I could be happy with either MATE, KDE, XFCE or LXDE and appreciate the different strengths of each (and indeed I have used all of these four extensively), and I could live with Cinnamon if I really had to although I wouldn't like it. However, for the non-expert user, MATE (Gnome 2) is still very clearly the best choice. Hopefully, with Mint 13 LTS, MATE will be supported for a long time. (On my own systems, it's going to be LMDE with MATE though.)
A big thank you to the Mint team for integrating MATE into such a very user-friendly distro!