I'll try to have a bash at answering your questions.
What was the quality of play like?
The standard was excellent. The difference in different levels of competition in any sport is the speed of the game. It would probably be correct to think that the Sydney comp was slightly faster - the play-the-ball speed of NSW often undid Qld in State games. The BRL competition at any time contained the future stars of the game, current State and Australian players and a fair number of past representative players. These were distributed across only 8 teams - not 12 like Sydney. Thus the standard, particularly of top four or five teams, was very high.
What were crowd attendances like?
The match of the round at Lang Park would typically get 15000 - 25000 and the other three games around 4000 - 7000. Grand Finals were a sell-out with 38000 cramming into Lang Park.
Was it more popular than NSWRL in QLD (like, was there more publicity)?
No comparison. Almost all media coverage - televison/radio/newspapers was on the BRL. Into the early 70's, the Sydney results were found in an obscure corner of Monday's paper - certainly no mentions on radio or TV. The newspaper coverage of the Sydney comp probably increased a little through the late 70's / early 80's.
What would you say the best games you attended were and best players were?
Stack of memorable games. My favourite was probably the 1975 Grand Final with Wests beating Redcliffe 26 - 24 after leading 26 - 9. Outstanding quality of play and a rivetting match - watched it again lately and it certainly stands the test of time. Too many great players to mention each club had its greats : Valleys - Scanlan, Threlfo, Strudwick, Lewis; Brothers - Gleeson, Manteit, Wright, Quinn; Redcliffe - Pearce, Leis, Obst, Beetson; Wynnum-Manly - Trevor Neibling, Hornery, Scott, Miles; Norths - Metassa, Kilroy, Khan, Lobegeiger; Souths - Vola Vola, Belcher, Astill, Meninga; Easts - Morris, Morris, Denman, Lang; Wests - Muir, Connell, Orr, Carr.
That's just picking out a few names from the top of my head.
What was tv coverage of it like back then (also, did QLD ppl get to see games from NSWRL comp or not)?
Replays of games from Lang Park started in the late 60's. Television news would cover BRL news every night of the week and it was a major segment of weekend sports shows.
Who were the commentators of the matches?
Most memorable were George Donniger and Mick Veivers. Fonda Metassa was one of the first, Arthur Denovan from the ABC, Billy J Smith was colourful, Warren Boland and David Wright started a bit later and are still going. Radio was the legendary George Lovejoy followed by Ross Lawson. Billy J, John McCoy and a few others came along later.
When did they stop televising the BRL/QRL comp?
They didn't. A Queensland Cup game is televised live statewide every Saturday at 2 pm. Rates extremely well.
When did its popularity/tv coverage/crowd attendances start to diminish?
Initially, I would say from the early 80's. State of Origin had taken a lot of the focus from the BRL comp once it started. Interest was still high but there were more other events to compete with. The BRL Grand Final in the 70's was the biggest single sporting event in Queensland each year - this was before Indy, SOO and even One Day Internationals. The quantum drop was in 1988 when coverage dropped from extensive to almost non-existant.
What's the difference between Brisbane Rugby League comp/ Queensland Cup/ Winfield State Cup and the Ch9 cup? (or are they all the same comp)?
The BRL competition featured teams essentially from the Brisbane region. It ran as the major competition until 1995, though they played two further BRL Grand Finals in '96 and '97. The QRL introduced the Queensland Cup as the major comp in 1996, initially featuring the Brisbane teams (except Valleys which "went broke") and teams from regional Queensland plus Port Moresby. A number of teams have dropped in and out of the Queensland Cup since then with 11 teams in it this year. Channel 9 were an early sponsor of the Queensland Cup so that is the same thing.
The Winfield State League had various guises since commencing in 1982 - it started as a part season comp involving the Brisbane teams plus the regonal rep sides, lasting about 8 weeks. The Brisbane clubs then played the BRL comp for the remainder of the season. The State League is no longer played although there is a regional championship played at the level just under Queensland Cup.