Glow Mapping
"Enables emissive surfaces to 'Bloom' over foreground objects"
Whatever that means, difference is significant. Colors look much more alive.
This option requires Post Processing Effects option to be enabled.
Overbright Bloom Filter
If I understand it well, it makes very shiny lit objects spread this light further to its surroundings.
I wasn't able to catch it on screenshot.
This option requires Post Processing Effects option to be enabled.
Blur Filter Quality
This option determines quality of Blur Filter used for some effects like Light Bloom.
As you can see on low setting bloom effect is a bit blurry and of lower quality than on high setting. It is important in areas like Mirkwood where a lot of Bloom is used.
This option requires Post Processing Effects option to be enabled.
Bloom Intensity
With this option you can adjust intensity of Bloom effect:
Importance of this setting depends on how much Bloom is used in certain area you are in. If there is a lot of Bloom used then you will be able to see big differences by adjusting it. Useful setting for adjusting Bloom intensity to your preferences.
Note: This option requires Post Processing Effects option to be enabled.
Player Mesh Combining
This option allows to render players faster whichallows to have more players on screen at once.
There is no information on how many players you can have on screen with this option and without it, so it is hard to say anything here. It is impossible to test it.
3D Object Portraits
Allows 3D portraits for your characters and selected players, NPCs and mobs.
You can live without it but it is nice looking gadget and it of course it is better to see 3D dynamic portraits instead of blurry circle.
Texture Cache Size
This option adjusts amount of system memory that can be used for graphics resources.
I wasn't able to spot any differences in visual quality or performance.
Player Crowd Quality
This option controls graphics quality of characters when huge number of players are nearby.
Again, no information on number of players and what "huge number" mean. Impossible to test.
Refresh Rate
This option lets you choose refresh rate.
Set it to auto or to your monitor's refresh rate.
Sync to Refresh Rate
This is Vertical Synchronization option. It adjusts number of frames rendered and displayed by your monitor to prevent screen tearing effect. Keeping it disabled will unlock framrate which will allow the game to render as much frames as it can. Keeping it disabled in game of LOTRO characteristic (CPU bound) may increase stuttering, and also it will cause tearing, so keep it enabled.
Triple Buffering
Explaining Triple Buffering is not really sensible for this thread. Read about it on web if you are curious. It may give you 1 or 2 extra FPS so keep it enabled if it is not giving you any issues.
Conclusion
So here it is, tons of hours of work I made almost two years ago, now in clear and readable form. It should help you understand LOTRO's graphics panel and you should be able to choose optimal settings now. It should be very beneficial for because overall presets are really not too good and for example choosing High preset is not really High, majority of settings are on Medium or even Low.
Performance Guide is coming soon.