Considering Nature's Grasp without Improved Nature's Grasp as Rank 0/4, the average (mean) number of hits to proc is an infinite series of numbers of hits to proc, and is equal to the reciprocal of the chance to proc. No battle is infinitely long, but for long battles this is close to accurate; for short battles Nature's Grasp can fail to proc (except in the case of Rank 4/4, where the proc is guaranteed.)

   Mean Hits to Proc (Long Battle), assumes a proc occurs eventually

Rank	Odds of Proc per Hit	Mean Hits to Proc (Long Battle)
Rank 0/4	35%	2.86 hits
Rank 1/4	50%	2 hits (86% reduction)
Rank 2/4	65%	1.54 hits (46% reduction from Rank 1/4)
Rank 3/4	80%	1.25 hits (29% reduction from Rank 2/4)
Rank 4/4	100%	1 hit (25% reduction from Rank 3/4)

This gives a sense of the benefit incurred by ranks of this talent in terms that affect the battle, but the assumption of a long battle is flawed. A character using this ability will usually be wanting to use it to pin a melee opponent so the character can step away - wanting the proc to occur within a few hits - so a short battle view must be used.

To do a similar analysis for short battles, assume a battle length (# hits) and do a series sum to get a mean time for Nature's Grasp to proc, if it procs, and to see what the odds are that it does not proc at all within the number of hits selected.

   Mean Hits to Proc (Short Battle), assuming a proc occurs

Rank	1 Hit Battle	2 Hit Battle	3 Hit Battle	4 Hit Battle
Rank 0/4	.35 hits	.81 hits	1.25 hits	1.63 hits
Rank 1/4	.50 hits	1 hits	1.38 hits	1.63 hits
Rank 2/4	.65 hits	1.11 hits	1.34 hits	1.46 hits
Rank 3/4	.80 hits	1.12 hits	1.22 hits	1.24 hits
Rank 4/4	1 hit	1 hit	1 hit	1 hit

These results assume a proc. If a proc occurs, in a short battle the ranks invested are not nearly as significant to the length of the wait to proc.

(A proc cannot occur in less than one hit; the mean is not always representative of obtainable results. A proc will always require at least one hit.)

Failure to proc is always (1 - odds to proc) ^ number of hits.

   Failure to Proc (Short Battle)

Rank	1 Hit Battle	2 Hit Battle	3 Hit Battle	4 Hit Battle
Rank 0/4	65%	42%	27%	18%
Rank 1/4	50%	25%	13%	6%
Rank 2/4	35%	12%	4%	2%
Rank 3/4	20%	4%	1%	0%
Rank 4/4	0%	0%	0%	0%

Odds of failure to proc within a tolerable number of hits shows a good progression distinguishing the investment in ranks of this talent. This is the key feature to look at when selecting an investment here.

Regarding "Four ranks of Improved Nature's Grasp always procs on the first hit." - is this true if the druid that cast Nature's Grasp dies from that hit? This would be significant in team play.