I used to race in the UK and back when I was a lad I was fairly handy, then a love for beer took over and then I moved to the US. Then there was more beer and bbq - ah, the joys of real steak instead of some grey gristly sh1t that has the consistency of tough boot leather.
A few years ago, just for ***** n giggles, I decided to get Cat 5 out of the way and stuffed 10 races into a few weekends. I weighed about 25lbs more than I did back then (low 140s Vs mid 160s) and my FTP was down about 25 watts (lab tested Vs PowerTap) but my ability to handle repeated anaerobic efforts was nowhere close to what it was... Fields varied in ability tremendously. You'd get some guys that sandbagged each year and purposely only did 9 road races within a 12 month period - so you'd be sitting on 300 watts holding wheels in those events, or you'd be dodging bodies of the folks that thought that kissing asphalt every 1/4 mile was a way to earn some cool road rash and some 'man points' in other events.
The biggest differences I spotted were more in bike handling skills. Maybe it's due to the lack of narrow roads, a ton of rain and crits held on roads wide enough almost land a plane on, bike handling here in the lower categories was ****. For many of the crits I'd rather sit on the outside edge of the bunch and catch some wind rather than catch a body on the floor when someone played dominos. The other big thing was what I'd call "the lemming factor" - the desire to go on the front and ride absolutely flat out for 45 seconds and then go bang, only to be replaced by someone else on the front that'd do the same. Fields thinned out in some events fairly quickly but it'd make for an interesting first 15 minutes. With Cat 5, you finish the event in one piece you get your point and rack up 10 points and you're upgraded. The events that were mainly Cat4's were smoother and seemed to lack very hard accelerations.