I'm not sure how many times these have been suggested or if they have been at all, but here are a few simple suggestions that I think would be useful.
Spell Pen. - Short and simple: show the character's spell penetration in their character sheet. While requiring us to watch the dice is not by any means difficult... it is unecessary and my suggestion should be a simple addition that would make things more convenient.
Caster Levels - Show both the current max caster level and current actual caster level on the character sheet. There is a difference, and the current way that your caster level is shown (in combat log) is not the actual caster level affecting your spell damage. Mine currently says I'm casting at level 31, but through experimenting it is obvious that is not true as removing gear that increases my caster level does not affect my average damage output until I am below level 25 on my combat log. This would help clarify some game mechanics for newer players or those who don't fully understand how caster level and max caster level work. UPDATE: Because caster level varies with spells, gear, etc. it would be a better idea to include caster level on individual spell tooltips similar to the way a character checks their spell DCs. This was suggested by Brennie
DPS - I believe that the training dummy on guild ships should also be able to calculate a players dps output over the period of time that it takes them to destroy the dummy. Unfortunately rogues and any other character with sneak attack would not be able to see their true dps because, for the most part, training dummies are immune to sneak attack. "For the most part" because the occasional attack may bypass the dummy's fortification to allow for a sneak attack. An alternative to this would be to display the weapons' average DPS in the details section of the inventory. This would then allow for sneak attack to be included in the calculation.
Alternative for Previous Stat Suggestions - Rather than changing the character sheet, I think a possibly simpler solution to show spell pen, max caster level, and actual caster level would be to create a new text command such as /stats that would show a more in-depth description of a charater's stats.
Useful Suggestions by Others
Doublestrike %: Show on character sheet or in a similar stat command or panel. Suggested by Durnak
Threat +/-%: Show both the melee and ranged/spell threat generation increase/decrease due to gear, enhancements, etc. Suggested by oradafu
Healing Amplification: Show a characters current healing amplification. Suggested by oradafu
Please add any similar suggestions you may have that relate to stat display and DPS measurements. I believe the above simple suggestions would help improve gameplay and convenience.