I suppose I could try doing the math myself, but I'd rather ask you guys, so here it is:

I'm playing a Nord and going for a light armored fighter. I've leveled up the following: light armor, block, one handed, smithing, sneak (no perks though), archery.
So this begs the question: am I spreading myself too much, so I'll be crippled in a few levels? (I'm currently level 14 or 15).

A follow up: I've heard there's a level cap at 50, at which point you don't get any extra perks. So how many perk trees can one fill up? I don't ever intend to swing a mace, and I don't think I'll bother with smithing perks for heavy armor, but I intend to dabble in alchemy, and lockpick would be nice as well (feel like I'm missing out on something when I don't open every door and chest).

Is it possible to fill up the following perk trees by level 50: light armor, block, one handed (no mace), smithing (no HA), sneak, archery, alchemy and lockpick?

I'll tell you this much, I'm level 48 with perks in Destruction, Alteration, Restoration, Heavy Armor, Smithing, Light Armor, Sneak, Archery, One-Handed, and Block. I can preform any role perfectly. There's no such thing as "spread to thin"

I am level 79, I have 4 full perk trees filled and 4 kinda filled out of the 18 total, which kinda svcks. you can fill those perk trees but, only 4 of them fully

I think that you would do just fine. The skills you listed do improve your combat potency, so there shouldn't be too many problems. You may want to eventually decide whether or not to stick to archery or melee, but other than that, I predict smooth sailing.

Thanks guys!