I feel really sad when I think...

...about how far I could have gotten by now had I just chosen a main and kept it. I have four characters in their 30's, one level 44, and about 5 in their 20's. I could have gotten a character to 90 by now!

And yes, I started when MoP started.

Did you have fun? I hope so!

If you only consider World of Warcraft in the context of how 'far' your main character gets, then that could be a recipe for needless frustration. Personally, I find the time I spend with my alts is well spent. I enjoy questing and doing lower level dungeons, and I find it fun to learn differing class mechanics.

With all your time spent experimenting and leveling alts, you've now got a lot of characters that you've gotten a feel for, so you're in a good position to pick one that you'll have a lot of fun playing when you get to end game. You'll also have a bit of a head start when the time comes to start leveling those characters up too.

No need for regret!

I used to love levelling lots of lowbies, it's good fun to try out different classes and specs!
The new talent system killed that for me though.
Levelling up with no talents to put anywhere really takes the fun out of it for me. Gaining a level is just gaining an arbitrary number.

I thought I might feel the same way, but now I find myself looking forward to the major milestones, like my next talent choice, important class abilities, and glyph unlocks. I find that I much prefer the occasional 'big bump' over more regularly spending a new talent point. Mostly because, generally speaking, those spent points didn't have a huge impact on my character. Each talent tree usually only had one or two major play style changing abilities in it. Now I feel like I get a lot more of those over the course of a character's leveling experience.

Believe it or not, that's a big part of what's motivated me to go back and level up some of my old alts that have been sitting around collecting dust.