You can't really look at content that way, as the ability to solo or group or raid any content has alot to do with gear, AAs, and tactics, as well classes involved. It also depends on the current state of the game, since as the game progresses, better equipment is available at all levels, thus making previous content much easier. The same is slightly true of AAs, but not nearly as much. Also, there are differences in tactics needed for different areas, mob types, and levels.
Overall, the amount of content in EQ is huge, and the way you approach the game has more to do with what's viable for you.
Just to give an idea of the vastness, the number of zones for each five levels as listed
here 1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
82 100 120 131 141 148 158 166 174 197 203 219 247 337 402 457 238 177
Alot of those are instances, and city zones generally range from 1-75, if not higher, so may disregard those. So even if you cut the values by one fourth, that's still alot of places to adventure at each level range. And a vast majority of the lower levels truly solo-able, and with mercs, many spots upto 80 or 85 duo-able with mercs. At the highest end, groups and raids are much more necessary, and groups can be found from around the 60s on up fairly easily if you really look / form your own groups, if that is what you are after.
Hope this helps some, but hard to explain as the vastness of the content is so huge, and what is requires, player-wise to complete anything depends greatly on the event, and the level, gear, AAs, classes, and tactics used by the involved players.
Yther Ore.
Edited, Dec 8th 2009 4:08am by Yther