Well, I guess I'll take all night talking about it.
For starters, I've been playing Starcraft ever since it came out. I was 13 during its release (August/September) and 14 soon after (October). I've been an avid RTS gamer all my life - first game was Age of Empires 2, later Age of Empires 3, the original Starcraft, Warcraft, etc. I played at a local LAN in Vancouver at the age of 14 at diamond league and reached masters league at 15 years old. I'm 18 now, and have since "quit" Starcraft, in a sense.
I see so many people here saying that "those who quit don't really like SC2" anyways. They're so wrong, in so many ways - I've spent bus rides staring at Starcraft apps, to the point of memorizing how much damage every single unit can take, how much health each unit has, doing battle simulations on my head. I drew pylons whenever I was in class and thought of SC2 non-stop. I dreamt Starcraft dreams and played SC2 exclusively everyday. Even when LoL was on the rise, I played SC2 and actively promoted it. I played during class (in CADD) and I played during lunchtime. I did exhibition 1v3s against my noob friends at school to show what SC2 is capable of so others would be interested in getting good at the game. I watched SC2 religiously and was super excited for HotS, snuck on to watch Blizzcon during class, and could describe any player in the GSL from personality to playstyle. I loved Starcraft II back then, and I still do now.
But I quit. The reason why I quit was simple - Blizzard stopped caring - about the game, about the community, about the player. I think the beginning of WoL was great - we had MarineKing with the name Boxer living up the legacy with great marine control, we had the Polt vs Jinro 'drama' and the foreign hopes in both Idra and Jinro. It was a golden time for Starcraft. Probably the greatest time was 2010 Blizzcon Finals with Mvp vs Nestea, with nukes on Shakuras Plateau - I remember the hype, and John the Translator's speech.
Then what happened? 4gates for PvP throughout the entire history of WoL. The imbalanced time of 1/1/1s. Later in WoL, Broodlord Infestor turtling. Protoss turtling.
Remember when Sniper won a GSL, after doing mass infestors and mass fungal against Ryung (refresher: IMBA IMBA IMBA IMBA IMBA), somehow coming back from an impossible game? Remember when the top 4 in multiple tournaments were all Zergs? Remember when Blizzard patched that only... 3 months later, despite nerfing Ghosts snipe damage two days before one of the biggest MLGs? The inconsistency in balance patches resulted in perhaps one of the longest one-race-dominated time in Starcraft's history, that being Zerg at the time. But there's no need to dwell in the past - this was the past, surely after the community outcry, Blizzard has gotten better at listening to the players, right?
Nope.
The community has told Blizzard what we want multiple times. I'm sure Blizzard knows the importance of community feedback. Not every idea is great, no, but if everyone agrees with an idea, it's generally a good idea to add it for the players so they'll be happy. Blizzard takes a slightly different approach.
We have an extreme amount of posts detailing the design failures of the Battle.net 2.0 interface, to no changes. We've asked that you can name your lobbies like the original Starcraft - nothing's changed. Make the game feel more alive with auto-connect chat lobbies and so forth. Nothing. We want an Abathur voice pack, nope. Unit skins? They added them, yes - and never added any new ones since the beginning of HotS. Clan wars? Nah, we'll wait till the next game. More team game support - nope. Ban maphackers. Still no official reply. Swarmhosts are boring. Only took several months before they addressed it, and they're still working on a fix! We've told Blizzard a ton of times we're willing to throw our money at them in Starcraft if they're willing to add unit skins or voice packs - and they're not doing it! I'm done waiting for Blizzard to have their interface have as much features as their original game from '98, and I'm done waiting for Blizzard to "ban in waves" (or so they say), and I'm done waiting for Blizzard to reject my money then report to the higher ups "Starcraft can only make money on initial sale".
While this next point may be up for debate, I would say that the player experience is the most important aspect of any game. If you look at Blizzard as a Hearthstone player or World of Warcraft player, you really feel the sense that Blizzard cares about you as a customer, and actively tries to make the game better for you (whether they actually do do that or not, that's a different story). That's why they changed the auction house in Diablo III. That's why they're adding expansion packs for Hearthstone and a new expansion for WoW every two years that changes everything up with balance patches, completely new content, and so forth.
As a Starcraft player, do you know what I get? Late balance patches. Maphackers ravaging the ladder. The idea that I have to win 50 games in order to get placed in freakin diamond. An interface that hardly connects me with anyone. "Clans" that can't do anything outside of make announcements for clan wars which are hosted in lobbies that aren't customizable. A poorly run WCS with late rulesets after the tournament already started.
On top of that, the fact that I have to win 50 games in a row to get back to platinum from gold is absolutely sickening. I don't see how getting a player to commit an entire day of grinding pointless ladder games was a good choice by them, but they continue to stick by it. If anything, it's a greater deterrent for any ex-diamond players who wanted to play one or two ladder matches before dinner (eg. me), because they know that those games aren't even going to be a challenge. Furthermore - the fact that they spent time on revamping the MMR system (which wasn't even originally terrible, just a couple minor tweaks would've done it) only meant less time they had to work on just about everything else the community asked for. Priorities, Blizzard.
The game isn't dead by any means, no. Starcraft still has the greatest number of interesting personalities and drama, there's still tons of tournaments, and lots of pros still play it. But I can't help but get frustrated as a player, at all the decisions that Blizzard has made over the years - the lack of support, the late patches (that were sometimes pointless, like eg. bunker time 30 -> 35 -> 30 -> 35 or oracle speed changes four times), the lack of care and quality that Blizzard is known for. It frustrates me to no end that Starcraft, the game I love, could've been so much more - that being good at the game and the ridiculous amount of skill and practice it takes actually meant something; that it could've been the number one game on Twitch in terms of viewership; that people would flock to stadium to watch one of the greatest, most competitive, high stakes game ever created - but Blizzard just flat out didn't care. Originally a diehard fan, years of Blizzard's wishy-washiness and lack of commitment to Starcraft II just makes me feel hollow inside.
Yes, I agree with Nazgul's post as in the community should not be so negative all the time. "ded gaem" is not contributing positively in anyway. But I do believe that criticism, in general, for Blizzard is needed - in every game, the ones that are the biggest fans should be the biggest criticizers. I believe that over the past few weeks or so, Blizzard has been trying - their latest post was certainly a reassurance that they're at least attempting new things - but if they don't continue to work on the game even after Legacy of the Void comes out, I don't see SC2 lasting for the long run.
TL;DR - I don't feel like Blizzard cares about what Starcraft II is up to, and couldn't care less if it dies. Their stubborness in some of their design philosophies, late patches, constant problems, map hackers rampaging ladder, inconsistancies and not really caring about the community makes it so I just don't want to support a game with all those problems. I mean, even Riot, with much less money, posts lore stories, balance patches every so often (with a complete redesign every season or so), things fans have made, host art contests, and generally attempt to get feedback from all their players - and uses their feedback in future patches.
I acknowledge that I've been wanting to write this for a while, and as such my bottled-up emotions may have led me to state certain things with a slight bias. I believe that my points, generally, are objectively true but of course, my opinion of Blizzard is an opinion. I just want to speak for the players who quit for similar reasons like mine, and perhaps if Blizzard is reading this, do something about it. Because seriously, I really want SC2 to succeed.
note: I still play this game, but casually with friends in 2v2s and 4v4s. I also watch it quite regularly whenever there are some good names.