When I first came back to the fold, I had picked up the idea that not eating meat on Fridays was something that Catholics used
to do, but Vatican II had done away with all that, because it really wasn't very practical... and anyway, quite a lot of the world's population never ate meat at all due to poverty, and so not eating meat on Fridays was a bit pointless...
Of course, no-one ever pointed out that some other form of penance was supposed to be substituted... I discovered that snippet of information some time later. The problem was that deciding on a suitable Friday penance was soooo hard, as I usually didn't remember until the end of the day, by which time the opportunity for all suitable penances had pretty much expired... "Well, I haven't eaten any chocolate, so I'll make that my penance..." doesn't really cut the mustard.
It was actually through reading blogs that I discovered that abstaining from meat was still considered to be a requirement for Catholics in some parts of the world. Then I discovered that some people still observed the penance anyway, without any "requirement" and I thought I should try it.
It certainly reminds me that it's Friday: I loathe vegetables from the very depths of my being (potatoes don't count as vegetables, especially when chipped and deep-fried), and I'm not much of a fish-eater (unless you count fish fingers) so not eating meat is definitely something I notice.
Some people will still object that, if you do like fish, then substituting a nice salmon steak (or whatever) for your usual pork chop isn't much of a penance... this, however, is more a demonstration of how they view penance than anything else: it's not necessarily something unpleasant, or even terribly difficult. Instead, it's about doing something for the love of God, making an effort for His sake.