What was Carl's problem?

It's a description which is nowhere near as accurate as it needs to be to get away with its own flamboyance.

Carl looked kind of gay to me, judging by the way he dressed. Maybe it's a repression & denial thing.

I used to want to change the world. Now I just want to leave the room with a little dignity.

You make some interesting observations Hugh1971, and I even share some elements of your general comments. I too run into a fair amount of 'Carls' myself and by-and-large have developed a strategy to avoid having to face a lot of their worst excesses. However I'm not sure this what Mike Leigh, or actor Martin Savage want us to take away as an explanation for Carl, although I can't be 100% certain of this. You may have a point if we consider that Carl can only express himself through gestures of hostility, insensitivity and contempt and such people, and the environment that makes them, do not always fare well in a Mike Leigh film. In an interview I saw recently, Leigh makes only a brief comment on him as something of a hit-and-run character, even though we're being prepared for Carl early on when referred to in earlier scenes.

Savage on the other hand has been, third hand admittedly, reported as saying he imagined that Carl might have been a lot closer to Tom & Gerri at one time, attracted by their easy going personalities and more sophisticated London lifestyle. His resentment therefore may have come from being cut adrift for some reason and left to continue life with his blunt father in a Derby back street.

I have to confess none of what Savage allegedly says occurred to me whilst watching these scenes. I locked more on how Carl seems to imply Ronnie had not treated his dead mother well, even if it was a just a convenient stick to beat his father with.

However this is just the sort of surly character one sees slouching about on a Jeremy Kyle set.

"I'm staring at the abyss.I don't like the look of the abyss"

Garp's original post and yours here gvyti very good insight to Carl's character. If you pick bits of dialogue here and there you can put a picture to Ronnie's character. The fact that he had all the time in world to attend football matches (unemployed), always at the pub, never shouted the drinks, Linda who must have been close to 70 was still working full time at the bakery supporting him and Carl. Ronnie saying to Tom, "I don't know what to do?" was meaning, who will look after me now and not about Carl which Tom misinterpreted. Mary would be in for a big shock if she thought she could wiggle in from this side of the family.
Gerri's look and not answering Mary when she said that Ronnie was "nice" was because she knew what Ronnie was like and how he treated his late wife.

BlueSkies

Another clue to Carl's aggression towards Ronnie for not treating his mother well, is when Mary asks Ronnie if Linda was "nice". He doesn't answer, but instead stares blankly and sips his tea. This could indicate that he had little feeling towards Linda one way of the other (hence Carl's anger), or possibly that he was too grief stricken to talk about her, especially with someone who he'd only just met (Mary).

I got the impression ronnie was completely emotionally inaccessible which must have taken its toll on the mother and carl over the years .

Carls anger was in huge contrast to joe's close relationship with his parents - another way of leigh comparing how unfortunate lifes can turn out when the family unit is not a supportive and comforting place or when one is cut adrift.