I've been through this:
-Mind the work life balance, small companies could easily get you on the hook for late nights and consistent weekend work.
-Get your job title and expectations in writing. If you're an architect, you're taking on a mentor and even a managerial role to an extent. This needs to be in writing.
-Who do you answer too? If the company is really small their structure might not be firmly established, which will cause issues.
-What is the atmosphere like? You may be coming from a peaceful work place to one where co-workers badger each other, are stressed out, over worked and have an unwritten 'pecking order'.
-Consider getting an ownership stake in writing for after you've been there for a 6 months - year, increasing thereafter.
-Who are their customers and what is their revenue? If they loose their biggest customer what happens?
Finally -
-Most small companies fail however they all have big dreams. Be careful.