He rode the wave of goodness that was created by Kalgan and was the first one to slowly ruin the game. Don't ever forget he is the one that gave dispel to every healer and removed it from sp/ret, handed out self heals like cookies and started the road towards complete homogenization we are on right now.
most people refer to WotLK as the best expansion
WotLK is also the expansion that had the best sub numbers/statistics
Ghostcrawler was the lead systems designer for that expansion
wave of goodness created by Kalgan and Tigole was shit like mace stun and a meta that implied that each class will have only 1 viable arena spec (never actually happened and several classes were more or less unplayable in arena), pvp trinket that removed only 3 types of effects (until later in tbc), only 2 classes with 50% mortal strike debuffs (making them mandatory to have for a viable comp, outside of vipersting/manadrain "run you dry" setups that people figured out way later), 2v2s that lasted 4+ hours being an accepted part of the game, and various other amazing perks. i mean, PvE (both solo and raid stuff) had just as many examples of utterly terrible design
people really forget how actually SHIT vanilla and TBC were gameplay-wise. don't get me wrong, my best times in this game were mostly in TBC, but that doesn't mean the designs were actually good. it was a different time. this doesn't mean that Kalgan or Tigole are bad game designers either - someone had to do the mistakes to learn from.
also you have to understand what 'lead systems design' actually implies. for example, wow has pretty decent core gameplay, what sucks are mostly the systems - e.g. 4 difficulties of the exact same instance being a pretty fucking shit idea. or there being no team charters, which in turn allows you to just log on whatever class/alt you have and just instantly queue your fotm setup at no penalty whatsoever, which results in everybody just playing fotm shit exclusively. or s10 having good gameplay but being completely ruined by the matchmaking/ladder system around it - this is a legit mistake you could credit to GC. but he did design a lot of good shit too.
its kind of idiotic to praise WotLK as the best thing ever and then go hate on the dude that was heavily involved in making it what it was. he's also the only dev to interact this closely with the community, and he's the REASON there's so many blue posts daily and devs just casually talking to us on twitter - he's the one who set this example. it must suck to give so much to a community only to receive a hate bandwagon in response. no wonder he left.