The aspirational wannabes and 'professional' housemates are less of a problem than the change from a collaborative to more confrontational character dynamic. That and the 'poisoned well' of outside information being revealed to the house.
In "Ye Olden Days" the general approach was to have a house that was for much of the time at least partly in consensus, where housemates would mix and mingle, form some kind of affinity with others and, yes, settle into groups (or cliques if you want to be pejorative), but that dynamic was allowed to grow and change organically as time wore on, people left and/or joined and tasks, deprivation and isolation brought out the best (and worst) in each other.
Nowadays Big Brother doesn't seem to have the patience for that, believes its ADD-generation viewers haven't either, and actively conspires to pull the rug out from HMs repeatedly (and IMO with a degree of malice) in order to foment discord, set people against each other and create rifts, divisions and arguments believing that shouting = drama = entertainment. At this point Big Brother is actually the
enemy of the housemates rather than simply the mildly dictatorial authority figure of old - the nasty mean bastard prison guard rather than the firm but fair warden.
The 'professional contestant' thing that people like to criticise isn't really going to go away I'm afraid, for a number of reasons. Firstly, at this point in the show's longevity applicants these days are not interested in "participating in a new and interesting social experiment", they're applying "to be on Big Brother" and are fully aware of what the show is and how it treats its inmates these days. Therefore, there must be more than the lure of a relatively small chance at winning a moderate (and by no means guaranteed) amount of cash to attract people to apply to be on the show. With the opportunities to parlay your Big Brother appearance into genuine fame and fortune even more proscribed these days than they were in the past (and TBH the chances of rising above the level of tabloid- and chat-mag fodder were never great even in the early days!) the remaining attraction is simply the opportunity to "be on the telly" and perhaps get invited to do PAs at dodgy provincial nightclubs and occasionally wallow in the same shallow (piss)streams of fame as the denizens of TOWIE, Geordie Shore, X-Factor etc.
Secondly, anyone can upload a profile to StarNow.co.uk or any number of similar 'talent' directories and a lot of the younger generation, raised on and envious of the lifestyles depicted in TOWIE and its ilk will do so, and these directories are precisely the sort of places harried production assistants got to to cast Big Brother and similar shows. You wind up with a self-selecting pool of applicants searching for some degree of public recognition and/or self-validation and the problem of having too many 'samey' individuals who are self-consciously trying to "make their mark" whilst simultaneously trying not to "poison their (putative) brand" by being an out-and-out arse results in a house full of 'performers' and a dearth of genuine emotion and drama that is then made worse by the artifice of the producers meddling to drive some other storyboaded narrative of their own.
So you end up with a surfeit of HMs who want to be famous simply as a goal in itself, the majority of whom are, on the whole, realistic enough to acknowledge that the kind of 'fame' an appearance on Big Brother brings is fleeting, trivial and very slight indeed, but hey, it's better than nothing and maybe it means you get to go to Sugar Hut and stand in the same smoking area as [insert name of orange z-lister here]. I myself randomly found myself at a Sugar Hut after party with Dean Gaffney, Dane Bowers, an ex-porn actress or two and a motley collection of assorted other folk simply because a friend of mine had a DJ gig there - No, I'm not proud but shit happens when you're drunk!
I believe that even (some) 'professional contestants' and serial 'reality TV' participants have the potential to be interesting and entertaining housemates if you get the casting mix broadly correct and let them "do their own thing" with substantially less manipulation by Big Brother or giving them huge steers as to how they're being perceived through "Who do the public think is the most...." quizzes and reveals of other HMs Diary Room secrets.
People are perfectly capable of forming and dissolving friendships, flirting, sulking, randomly causing chaos or getting on one anothers tits by themselves in what is meant to be an isolated and stressful environment, they really don't need any extra help from a Big Brother with an overweaning compunction to "light the blue touch paper and retire to the safe distance of the production gallery". The Danny Wiskers of the world are IMHO a minor problem by comparison.
I've always believed that 'Nasty Nick'-gate, way back in BB1 is probably the
apotheosis of genuine 'edge of the seat' organic BB drama as it was one of those occasions that just developed unexpectedly and producers had to react to it, whilst the BB Bedsit and the resulting Fight Night is its
nadir since it set in motion the whole train of tormenting HMs by revealing what others think of them just to "see what would happen" and deliberately, unnecessarily, stirring up trouble that has now utterly corrupted the show. Whilst BB5 and subsequent shows often had much to offer, if I'm honest that for me is where the rot began.