Keep in mind that there is no casino which is above the rest on all attributes (features). Some casinos offer awesome bonuses yet you will have hard time cashing those out. Others offer fast payout but their support leaves much to be desired. Yet other casinos with excellent support have a very limited selection of games.
A sound rating system should account for those multiple parameters and essentially calculate the average. Bryan has already told you how he calculates this average for the Casinomeister Rating. Here is my 2 cents on the important features for this casino.
General. Publicly traded (very few can boast it). Licensed in Gibraltar (not bad), established in 2002. Also has a poker room, a sportsbook, a bingo room (not only convenient but also indicates a solid operation). Web site is offered in 9 languages (again, an indication of them taking it seriously to make it player-friendly).
Software/Games. Not a multi-platform. Yet, Microgaming is arguably the best software provider out there. Hundreds of games both in the Download module and instant play, mobile version available. Live dealer games available. Games with 7 digit jackpots are there.
Bonus Size/ Percent. There are casinos with bigger bonuses. Yet, what 32red offers is not bad at all.
Bonus Terms. They have a 6.25 bet size limit for the welcome bonus, which is not good. Otherwise, very clear terms, winnings won't be voided for doubling on slots, or for playing a wrong game, or for making a "low risk bet". No universal gotcha "bonus abuser" clause.
Payouts. All major ewallets available, yet missing some local popular ones. 24-hour payouts and they do pay in 24 hours not just say it. No payout limits (not too many casinos can boast it, some have very low limits). Very reasonable document verification requirements.
Support. 24/7 Chat, email, phone (you may be surprised to know how many casino lack one of the three). Friendly and professional.
Complaint Options / 3rd party arbitration. There are casinos which have none of such, others offer very limited options. With 32red, should a misunderstanding occur you can go to Ecogra, to Gibraltar Gambling, to this forum, and at least 3 other portals I am aware of where they have a rep. Your complaint will be taken seriously and addressed quickly.
Web reputation. Like I said - they have reps on multiple portals, and all of these portals rate the casino highly. I can't think of any single site which has 32red in the black list
Player reputation. I have read dozens of player stories, and have not found a single one (!) with a grounded complaint which was not resolved. Vast majority of the stories are very positive.
Now you average it all, and score it against other casinos to obtain your own rating