Lets make a list of Pros and Cons. How about we start off with JRank's list and continue adding things.

Pros
I will beast in my age division
(Barring hax) almost free ride to nats
Chance of winning a trip to Worlds for masters division
Lower chance of competing against each other until we win something (buddy rule might not work from now on)
Faster tournament/tournament rounds (mostly for Regionals)
More slots for Nationals and Worlds

Cons
Won't feel like I did anything against twelve year olds.
Won't get to compete with the absolute best.
Will have skewed battling times for divisions, so less hanging out with Smogonites.
If I do well, people (older Smogonites) can be all "but you didn't play against the best. We can say the same thing if they do well
Later on some of the people in the senior division left alone. i.e. Cybertron / JRank

And I must clear up because someone thinks I meant something *coughCybertroncough* when I mean I did nothing against twelve year olds, I mean the twelve year old noobs. ;) (aka NOT Aaron)

I'll do better this year with all of the older players out of the way.
It sucks that this is the only year I get to be in senior division (born 1996), so I better make the most of it.
Many older trainers are tutoring younger kids too, so we'll need to watch out for them.
Most likely there will be less people in our division, so it'll be harder to avoid the good trainers.
My greatest fear is losing R1, which would suck major dick.
...I'm considering to pay my brother (born 1999) to lose against me on purpose R1, or pay a friend to do the same.

I won't have to face pokerob this year then! I'll admit, I wasn't using ubers, so meh. :P
Suppose I'll actually have a chance this year.
Although what annoys me is I had a chance last year, too.

I'm terrified of losing first round! I would NEVER live it down if I got nervous and a noob beat me. Would never live it down. Ever. :'|

.

the cons except for 3 and 5 are the results of like a napoleon complex or something. winning is winning is winning.

Loosing to Round 1 will be highly unlikely if you happen to make the right team. I'm not sure how many of us are making teams that can use trick room to their own advantage. The fact that our opponent might not even use those strategies might give certain counter teams a disadvantage.

well with team preview, it will be easy to predict the opponent's strategy (even easier if they only bring 4 pokemon in). just bring an imprison something if you're really worried about trick room :p

Team preview has taken a lot of stress out of battling for me, personally.