Warning, long response. Sorry for typos but I have been thinking about this a lot and I have talked to a lot of people over this season.
Thoughts for hitters: Top pitching from starters to closers. Not just velocity but location and secondary and tertiary pitches that are devastating. So there are no gimmee innings on inferior pitching in the middle of a game before the closer comes in. Only capitalizing on rare mistakes, adjusting to make something happen or catching a really good pitcher on an off day. Definitely no sitting on fastballs. They will just strike you out with something else. It is something my son is working to adjust to because he tends to look for something he can drive and the pitchers in AA can pitch undriveable strikes (if that makes sense).
Metal vs wood bat. Some players like wood but for most, it means less pop.
Many have to learn a new position.
Insane defense. Even moving from high A to AA my son noticed very little gets through the infield, very little drops in the outfield.
Thoughts for pitchers: Read the first paragraph and replace "starters to closers" with "top to the bottom" of the line up. Most of them can hit high velocity. You have to be able to do a whole heck of a lot more.
Way more speed on the base paths. It's somewhat balanced by the better defense but not completely.
There is a ton of depth on the pitching staff so you really have to distinguish yourself.
Thoughts for both: The length of the season. 132 in the minors vs. 71 for a team that competes in the CWS Final. Far fewer games for the normal college team. Pro ball you play 6 games, one day off, play six games. That's not counting spring training games, fall league, Dominican/Australian league games if you play in them.
You have to stay healthy through that long season.
Baseball is a freaking hard sport. Players who have never failed before will fail in the minors in some way or another. And now they have to fail away from their traditional support structures.
Trades and promotions. You could finish a game one night and have to play somewhere else the next night. My son has only been at it for two years and that has already happened to him 6 times. Three promotions, one trade and two injuries. Trades (leave all your connections, teammares/friends behind to an unknown) and demotions/IL are the hardest but even promotions are crazy. Get called into an office at 10pm. You're told congratulations, pack up, go somewhere else and play the next day at a higher level with new people who may or may not resent your presence.
College players have less time to "prove" themselves. There is a big difference in the eyes of execs (fair or not) between a 21 yr old struggling after 2 years of minor league baseball and a 23/4 year old struggling after 2 years of minor league baseball.
Someone talked about the funnel above but forgot to mention that you have to double the people going into the funnel when you add in international players. I would say half of my son's team is international and most of them were able to focus more energy on baseball at a much younger age.
Opportunity. Some teams promote from within and some teams trade for the MLB players. Some teams fast-track prospects and some teams slow walk. You might be playing great and you get screwed because there is no room for you on the MLB or even AAA roster. Versatility helps and for the most part they will figure out how to get a bat in the line up.
Not everyone is given the same chance to succeed. It's a part that's hardest for me, as a Momma, to see. I think all the young men my son has played with are great. He has already had one roommate and one really good buddy DFAed. Another good friend was moved down to instructionals. And every single year twice a year (international and national draft) a huge influx of new talent comes in. You are an expendable asset no matter who you are or how good you are.
Last thoughts: I couldn't find an updated breakdown but as of 2019, 58 of the 335 inductees in the MLB HOF played in college. Only 2 from the SEC. People like to say the SEC is like AA. It's not. It is like taking the top 20% of the SEC but the average player is 3 years older with commensurate experience, strength and mental toughness.
Final thought is conditions in the minors are significantly better than they were just 2 years ago. Current MiLB players are indebted to those players who risked their careers to make such a huge difference for future players.