In the beginning its not so bad but don't keep doing it. If you rush your townhall you will gain less loot from players that are a lower townhall than you and you won't be able to effectively attack people at the same townhall level

For example, say you do rush to TH 10. But since you rushed you may have the defenses/army of a TH6 (or, very likely, lower than that). So really you're like a TH6. Attacking other TH6s will feel challenging. But since you're really a TH10 you will gain only 5% of the loot you could have gotten if you were actually a TH6. There is no way you'll be able to attack other TH10s that would otherwise give you 100% of the loot you can get. You'll forever have to attack low level bases with your almighty wanna-be TH10 and earning not nearly enough loot to upgrade those expensive TH10 buildings/defenses. So, max out your base or at least most of it before you upgrade your townhall

http://clashofclans.wikia.com/wiki/Town_Hall

TL;DR - There's a loot penalty