1. Deadly Boss Mods + Bigwigs. One of either being a requirement.
2. Highly detailed boss guides all over the internet, including a good basic idea of boss mechanics for Sunwell being available before it hit live servers.
3. Highly detailed and well made boss guide videos all over youtube.
We didn't have the dungeon journal but it surely wasn't hard to find out boss info, what we know today from the Journal is perhaps only a convenience that slightly speeds up the process. Though boss strategy's could be quite complex back then the fights themselves didn't include much "player responsibility" compared to today, save for a select few fights.
I remember Tier 5/6 raiding and some of the hilariously bad players we had, I remember people failing to absolute basic mechanics and we still overcome the bosses, I remember this not even being unusual, the sort of player that gets made fun of and kicked from LFR wasn't all that unusual in my first experience raiding. It was only when I joined the Sunwell progress guild that I saw what a properly organised and focussed guild played like. This is a time when I considered myself an amazing PVE player and if you looked at damage logs (often the measure of skill back then) I was more than capable, but if you had thrown me into todays raids I wouldn't have stood a chance.
Raiding definitely became a lot more difficult on the average, I don't think dungeon journal really makes any real difference if you're comparing to TBC.