Currently a deterrent to grouping is that skills tend not to go up much as you level. People solo rank as a result, often the tanks.

Perhaps you could have a toggle where the boost from grouping goes into skill learning rather than exp gain? I suspect this would partially address the issue and lead to more grouping, so more fun.

It would be even better if it had some echo to let you know you
wouldn't have gotten this skill gain if you were not in the group.

There are a few things that could be (hopefully) easily changed such as:
1. a morale boost for killing mobs in a group. --hoping higher morale = more skill gains
2. Just a larger scaling bonus to skill gains for fighting challenging
opponents in a group or maybe just in general because groups usually
are going for stronger mobs. The risk vs reward for fighting more
challenging mobs has always felt off to me. Maybe that has to do with
the random nature of skill gains though.

As an aside to this: I feel defense skill gains should scale a lot
better with higher level mobs. Since you will be dodging/parrying
/shieldblocking them less it just is a little more brutal to learn
against higher mobs for it. Unless you have a healer.

Sorry for the length of the post; I wanted to include data.

My last two major characters were shamen, who I consider to be more on the tanking side. Looking at my leveling history with an Imperial duergar and Fortress dwarf, most of my levelling was done solo. Even at the higher ranks, I leveled solo when I felt I wanted to perfect (or at least improve) certain skills.

When ranking in a group, I often feel pressured to use skills/supps that will help the group learn, even if I've perfected them. Example: fatigue and demonfire vs. dispel and energy drain, which I really want to work on.

By ranking alone, I can work on what I choose. The downside is that I am not interacting with other characters, which is something I also like to do. I'm not sure how this idea affects game balance nor how much work it would be to code, but I think the intent behind it has merit.

Data:

"Daltheis's Leveling"

Leveling History
Jul 6, 2013 |Lv 2 |Hr 0 |
Jul 6, 2013 |Lv 3 |Hr 0 |
Jul 6, 2013 |Lv 4 |Hr 0 |
Jul 6, 2013 |Lv 5 |Hr 0 |
Jul 6, 2013 |Lv 6 |Hr 0 |
Jul 6, 2013 |Lv 7 |Hr 1 |
Jul 6, 2013 |Lv 8 |Hr 1 |
Jul 19, 2013 |Lv 9 |Hr 2 |
Jul 19, 2013 |Lv 10|Hr 3 |
Jul 19, 2013 |Lv 11|Hr 3 |
Jul 19, 2013 |Lv 12|Hr 4 |13 Pteraxul,
Jul 20, 2013 |Lv 13|Hr 5 |
Jul 22, 2013 |Lv 14|Hr 8 |
Jul 22, 2013 |Lv 15|Hr 9 |
Jul 22, 2013 |Lv 16|Hr 13 |
Jul 22, 2013 |Lv 17|Hr 16 |
Jul 23, 2013 |Lv 18|Hr 18 |
Jul 25, 2013 |Lv 19|Hr 26 |14 Scaguille,
Jul 26, 2013 |Lv 20|Hr 29 |
Jul 30, 2013 |Lv 21|Hr 43 |
Jul 30, 2013 |Lv 22|Hr 45 |
Aug 3, 2013 |Lv 23|Hr 54 |
Aug 4, 2013 |Lv 24|Hr 57 |23 Rogoza,
Aug 7, 2013 |Lv 25|Hr 67 |25 Nighden, 29 Rogoza,
Aug 9, 2013 |Lv 26|Hr 75 |
Aug 13, 2013 |Lv 27|Hr 83 |24 Rahzak,
Aug 15, 2013 |Lv 28|Hr 92 |
Aug 15, 2013 |Lv 29|Hr 96 |
Aug 19, 2013 |Lv 30|Hr 105|
Aug 21, 2013 |Lv 31|Hr 118|
Aug 24, 2013 |Lv 32|Hr 125|
Aug 27, 2013 |Lv 33|Hr 137|
Aug 27, 2013 |Lv 34|Hr 138|31 Zarozax, 29 Lalasi,
Aug 27, 2013 |Lv 35|Hr 141|
Aug 30, 2013 |Lv 36|Hr 152|
Sep 6, 2013 |Lv 37|Hr 170|35 Zukgakmuk, 41 Finley,
Sep 7, 2013 |Lv 38|Hr 171|
Sep 7, 2013 |Lv 39|Hr 177|
Sep 8, 2013 |Lv 40|Hr 178|43 Ruknubuk, 42 Nargel,
Sep 15, 2013 |Lv 41|Hr 187|38 Fegul, 45 Ruknubuk,
Sep 15, 2013 |Lv 42|Hr 188|39 Fegul, 46 Ruknubuk,
Sep 28, 2013 |Lv 43|Hr 203|51 Merf, 45 Nargel,
Oct 6, 2013 |Lv 44|Hr 217|51 Rahzeez, 40 Naldigar,
Oct 6, 2013 |Lv 45|Hr 218|41 Naldigar, 37 Taris,
Oct 6, 2013 |Lv 46|Hr 218|42 Naldigar, 38 Taris,
Oct 6, 2013 |Lv 47|Hr 219|
Oct 12, 2013 |Lv 48|Hr 225|
Oct 14, 2013 |Lv 49|Hr 231|50 Lyranoptil, 44 Sithani,
Oct 14, 2013 |Lv 50|Hr 231|51 Lyranoptil, 46 Sithani,

"Merrol's Leveling"

Leveling History
Aug 14, 2012 |Lv 2 |Hr 0 |
Aug 14, 2012 |Lv 3 |Hr 0 |
Aug 14, 2012 |Lv 4 |Hr 0 |
Aug 14, 2012 |Lv 5 |Hr 0 |
Aug 15, 2012 |Lv 6 |Hr 1 |
Aug 15, 2012 |Lv 7 |Hr 2 |
Aug 15, 2012 |Lv 8 |Hr 3 |
Aug 15, 2012 |Lv 9 |Hr 4 |
Aug 15, 2012 |Lv 10|Hr 4 |
Aug 15, 2012 |Lv 11|Hr 5 |
Aug 15, 2012 |Lv 12|Hr 6 |
Aug 15, 2012 |Lv 13|Hr 7 |
Aug 16, 2012 |Lv 14|Hr 8 |
Aug 16, 2012 |Lv 15|Hr 10 |
Aug 16, 2012 (100% exp bonus)|Lv 16|Hr 12 |
Aug 18, 2012 |Lv 17|Hr 14 |20 Berseial,
Aug 18, 2012 |Lv 18|Hr 16 |
Aug 18, 2012 |Lv 19|Hr 18 |16 Zjenavive, 18 Aiehymineya,
Aug 19, 2012 |Lv 20|Hr 19 |17 Wethrill,
Aug 21, 2012 |Lv 21|Hr 27 |
Aug 22, 2012 |Lv 22|Hr 31 |
Aug 22, 2012 |Lv 23|Hr 33 |
Aug 23, 2012 |Lv 24|Hr 36 |27 Zhederick,
Aug 23, 2012 |Lv 25|Hr 38 |
Aug 26, 2012 |Lv 26|Hr 50 |30 Fasstion, 30 Salvachnar,
Sep 3, 2012 |Lv 27|Hr 61 |23 Girensai, 30 Gisanithas,
Sep 3, 2012 |Lv 28|Hr 61 |23 Girensai, 30 Gisanithas,
Sep 3, 2012 |Lv 29|Hr 62 |
Sep 8, 2012 |Lv 30|Hr 66 |32 Isyndor, 40 Oshui,
Sep 8, 2012 |Lv 31|Hr 67 |33 Isyndor, 41 Oshui,
Sep 9, 2012 |Lv 32|Hr 70 |33 Silasalan, 34 Vontice,
Sep 9, 2012 |Lv 33|Hr 71 |34 Silasalan, 34 Vontice,
Sep 30, 2012 |Lv 34|Hr 96 |30 Jesp,
Oct 6, 2012 |Lv 35|Hr 108|
Oct 27, 2012 |Lv 36|Hr 125|
Nov 6, 2012 |Lv 37|Hr 147|
Nov 15, 2012 |Lv 38|Hr 164|
Nov 25, 2012 |Lv 39|Hr 178|
Dec 1, 2012 |Lv 40|Hr 195|45 Rahedra,
Dec 14, 2012 |Lv 41|Hr 237|
Dec 16, 2012 |Lv 42|Hr 241|36 Daerynus,
Dec 30, 2012 |Lv 43|Hr 272|47 Kalgranesh, 51 Filndar,
Jan 12, 2013 |Lv 44|Hr 293|
Jan 25, 2013 |Lv 45|Hr 314|
Jan 27, 2013 |Lv 46|Hr 319|51 Andira, 51 Oshui, 49 Khanion,
Jan 27, 2013 |Lv 47|Hr 320|51 Phansee,
Jan 27, 2013 |Lv 48|Hr 321|51 Phansee, 51 Khanion,
Jan 27, 2013 |Lv 49|Hr 321|51 Phansee, 51 Khanion,
Jan 27, 2013 |Lv 50|Hr 321|51 Phansee, 51 Khanion,
Jan 27, 2013 |Lv 51|Hr 322|51 Phansee, 51 Khanion,

Any chance for a user toggle of the bonus being skill learning or experience?

They went up much slower solo ranking.

It's just easier to solo rank with a melee class these days, and probably maximizes your potential to know your abilities.

It really sucks for some classes though.