If you're coming from oldhammer the first thing you need to know is that Age of Sigmar is a drastically different game. Objective based rather than kill based, and "looser" movement means certain critical aspects of oldhammer (wheeling, flank charges, etc) simply don't exist.
However, i've come to enjoy it infinitely more than i ever enjoyed WHFB (and i enjoyed that a lot). AoS moves faster, is more balanced, and seems more variability in effective strategies. As
@Krissey said, there's a ton of hidden depth to the game that make sit quite enjoyable competitively.
Seraphon are very competitive at the moment, and our competitive lists function a lot like they did in WHFB. Your strongest assets are movement, shooting, and magic, and you need to make use of all of them to win against harder lists.
However, don't fall into the all-to-common trap of thinking thats the only path forward. At a slightly less competitive level everything from melee-based saurus lists to stegadon-based monster lists can be effective.
Also, rebasing your models sucks but its very much worth it. Things look so much better on circles. Take that from someone who rebased 100 saurus and over 100 skinks (plus all the heroes, etc). Its a pain, but the end result was very nice.
I will say, if you do end up jumping into age of sigmar, there's nothing saying you have to play seraphon. Explore the range and see if something calls to you. Its got a new aesthetic and that might change what you like or don't like in an army.