Monica Gellar goes through some major changes in Friends - from a quirky chef who loves to clean to a married woman with twin babies and a house in the suburbs. It's a huge change, but a good one, as Monica has always wanted marriage and children. However, not every change in Monica's life is positive.
Over the course of the series, Monica also seemed to get more intense, less friendly, more competitive, and generally more mean-spirited - not exactly what fans wanted from the generous Monica of the pilot episode, who opened her home to an old friend who she hadn't seen in years!
She Went From Quirky To Scary
Initially, Monica just liked things to be clean and tidy, and wanted everything done a certain way. It was a quirk, but a funny one, and an understandable one, too. Plenty of people have dealt with a roommate who wanted things a little more organized than they did, and it was part of what made Monica unique. By the end of the series, though, Monica has become shrill, controlling, obsessive over tiny details (like the order of the fridge magnets) and just plain scary!
Her Voice Gets Way Too Loud
This goes hand in hand with the entry above - and while there is nothing wrong with a woman having a loud voice, Monica's voice went from fairly normal to shrieking, for no real reason. It was all part of the desire to exaggerate her high-strung nature, and make her seem even more intense... something that doesn't do the character any favors.
All The Fat Shaming
This isn't so much on the character herself, but on the writing - and it just isn't funny. Early Monica was just... Monica. However, as the show progresses, it's revealed that Monica 'used to be fat'.
Rather than dealing with this with any level of sensitivity, or making it a useful part of her character (dealing with bullies, insecurities, being suddenly perceived as conventionally attractive, etc), it's just a punchline, and it cheapens her characters.
She Goes From Naive To Bitter
In the early seasons of the show, Monica wants to find love - and she gets hurt doing so. From Paul The Wine Guy (who lies to her to get her into bed) to her later dating mishaps, it takes Monica a while to find her 'true love' in Chandler. However, as the show goes on, she becomes increasingly bitter about it, and starts to make mean jokes. It makes sense that getting hurt might make her more cautious, but this takes things to an extreme that just isn't fun to watch.
She Seemingly Abandons Her Job
It's something of a running joke (and even referenced in the show itself) that the gang seem to have an awful lot of free time to hang out at a coffee shop for people who are seemingly employed full-time, but Monica seems to abandon her job in the later seasons.
Early in the show, she is seen in her kitchen, she talks about work, she launches a catering business, she deals with reviewers - in short, she's ionate about her work as a chef (even if she seems to work wildly unrealistic hours). By the end of the series, though, she barely mentions it. Even when having babies and moving to the suburbs, what happens with her job is hardly brought up, and to see a woman who was so happy with her work just dump it isn't a good look.
She Starts Treating Her Parents Like A Teen
Unlike many of the Friends gang, Monica and Ross's parents actually become more of a presence in the show as time goes by - which is great to see, as it helps to flesh out the characters. Except that in Monica's case, it seems to make her revert to a sulky teenager. In the early episodes, she seems to have some level of deeper relationship with her mother - complex, but real. In later seasons, though, it's all about the punchlines, and Monica's actual relationship regresses.
She Becomes Obsessed With Marriage And Babies
Monica has always wanted to get married and have kids, and there's nothing wrong with that - but as time goes by, she goes from a woman who knows what she wants from the future... to a woman obsessed. When she gets engaged, she goes into full-blown Bridezilla mode, demanding Chandler spend his entire savings on a wedding (and even before that, she books a venue behind his back!). When they decide to have kids, she lies and manipulates him into sleeping with her when she's ovulating - all in all, it's not a good change, or an acceptable thing to do.
She Gets More Controlling As The Show Goes On
In the early episodes, Monica is still someone who wants control of her life, and bemoans the fact that she doesn't have it. However, this doesn't really spill over into trying to control other people until later in the series. As things progress, she becomes so controlling that Chandler is quite literally afraid to annoy her, and will say, do, or wear whatever she tells him to. It's just not cute, at this point.
She Turns Into A Caricature Of Herself
This has been touched on already with her cleaning obsession and her controlling nature, but as the show goes on, the other elements of Monica's personality start to fall away, and she becomes single-minded and a caricature of herself. The early seasons Monica was vulnerable, hopeful, career driven... and yes, she was quirky and Type-A, but it all balanced out. When the show started focusing entirely on the quirks, though, they lost the balance.
She Starts Lying To Her Friends
One of Monica's greatest traits is how good she is to her friends - she takes Rachel in during the pilot, even though they barely talk any more. She's kind to everyone, a great hostess, and that sweet side of her nature balances out the slightly less enjoyable parts. However, as the show goes on, she becomes less and less honest to the people around her - and less legitimately kind (and more obsessed with the appearance of kindness). She lies to her friends about dating Chandler, and later when Phoebe and Rachel have to move out of their apartment due to fire, she isn't so much kind as weirdly smothering. Seeing the best of Monica fade away like this was just sad.