GMod 13 was cited as a bad idea from the get-go.
12 was fairly stable, the addon system was great and overall everybody was having fun. Yes, there were a fair few Lua Exploits, yes Steampipe and the newer Source Engine game content wasn't available, yes Workshop wasn't enabled (and hooray for that actually), but overall the game experience worked fine.
13's introduction broke a lot of features that shouldn't have even been touched, not including the addon system. You can check YouTube for a number of videos from big machinmators about how their workflow got screwed up because of the unnecessary changes that were made..
13 is much better now, but I'm not satisfied like I was with 12. I lost all of my original addons and saves, plus all of the Counter-Strike SWEPs (for some reason Garry's Mod won't let me use my Counter-Strike weapons even though I've mounted the game) and a number of other props. I'm not playing GMod for the game-modes, I'm there for the content, and every update that breaks content (not even modded content) probably needed more work before being released.