I posted this on the front page but I know a lot of you probably don't read the comments there. The first part is a math proof, so if you want to skip it find the bold-ital-underlined tl;dr further down.
Ok, for the mathematically inclined reader:
Let n be the number of points you get through your character’s “career” all totaled. There is no way you can get more than n.
Let k be the number of points you can put in any one skill. There is no way you can put more than k points into a skill.
Then if only 3 skills are cap-able, the other 4 must not be capped and must have r = (n-3k) points among them. It must also be true that
r < k
for if we assume r >= k then we could dump k points into a fourth skill, and our assumption that only 3 skills are cap-able is wrong. Proof by contradiction.
This means that no matter what, you’ll have to pick from something approximating one of the following:
Jack-of-all-trades: All of your skills have something like 8/16 or 9/16 points.
Specialist-1: 3 capped skills and one skill with something like 12/16 points, and only 4 skills total.
Specialist-2: 3 capped skills and one skill with 9/16 and three with 1/16.
Specialist-3: 3 capped skills and four with 3/16.
Granted I pulled the specific numbers out of the air, but the math shows that if you only have 3 cap-able skills, you can’t even come close to maximizing the non-capped skills. You will be forced to make some very serious trade-offs in terms of relative skill strengths. Either play a specialist with “one-point-wonder” utility skills as backup, or generalize at half-cap in all the skills.
I can’t stress this enough: I just proved mathematically that you can’t do better than approximately half in all skills, or cap half the skills and mediocre points in the rest, or somewhere in between the two extremes.
The whole argument is moot if skills have variable caps, e.g. slow time caps at 5 and arcane orb caps at 16, but this will be difficult to keep a hard-coded “7 skills at most” and still ensure that you spend all your points. I can’t imagine they would ever want you to purposefully build your character so that you don’t use all your points. I won't go into all the different mathematical ways "7 skills at most" clashes with "can only cap 3 of 7 skills" and "variable caps on skills" because there are a lot of things to consider, but suffice it to say it would take a precarious balancing act to get all three of those propositions to hold at the same time, especially since there are 25 total skills per character (approximately).
My opinion based on the above facts:
There will be serious diminishing returns on most, if not all, skills. This will prompt us to not cap three skills unless it is truly a power-house build and really suits our play-style. That is, we will be "guided" towards playing a generalist character with most skills at 8-9/16 instead of capping three skills, unless the latter works better for our particular playing ability. This means a lot of customization options to accommodate different play-styles and different players, which iirc is one of their stated goals (or will at least earn them the most money).