What are some IP-related mistakes easily avoided by startups?

All the other posters have made great comments on trademarks, domain names, etc., which I agree with. Here are some other thoughts:
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A number of common mistakes occur in the intersection of IP, market analysis, and security. I have seen numerous startups launch into what they considered a good idea without doing a detailed enough market analysis to ensure that (a) what they propose is unique and useful, (b) what they are doing does not infringe on other companies' IP, and (c) they can create and maintain a defensible market position.
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Some investors, particularly in software / social media, no longer believe in patents as a form of protection because of the short time horizons for success of the business vs. the length of the patent prosecution process. Others, such as investors in hardware companies and healthcare, view IP as a key means of protecting a nascent market. This includes trademarks as well as patents; I am only addressing the latter here.
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Caveat on what follows: I am an inventor / serial entrepreneur, not a patent attorney. As such, what follows is a matter of opinion, and not advice.
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In the USA, I have seen many people falsely believe that they can protect their ideas with a hastily prepared provisional patent application written by the founders. While nice in theory, the reality is many provisional applications are of the form "Wouldn't it be nice if we could do X, Y, and Z?" rather than of the required form "We disclose a novel system and method for performing X, Y, and Z." Without the disclosure that shows novelty and non-obviousness, as well as enablement (providing enough disclosure that someone skilled in the art can reproduce the invention without undue experimentation), the provisional is worthless. In the end, this comes down to the question of can we write claims against the provisional specification, without having to go back and amend it to describe terms of art, enabling technology, novel steps, etc.? In many hastily prepared provisionals, the answer is no, so then the application must be completely reworked and the priority date of the provisional is lost. I have had to deal with this on several occasions.
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Much better, then, to work with a patent attorney from the very start. If you must file a provisional (usually because of impending product launch or other disclosure), you must make sure you can write claims against it. The most foolproof way to do this is by actually writing the claims you want, and using a concordance diagram to show that every clause of each claim is supported by exact language and figures in the specification. If you're doing that, you might as well file a non-provisional application rather than the provisional one. As someone who has filed over 30 patent applications, I can say I would NEVER file anything, even a provisional, without the aid of a good patent attorney.
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Finally, in order to keep a defensible market position, there are undoubtedly trade secrets (keying material for database encryption, customer lists, algorithms for matching vendors and clients, etc.). Often, this material is not adequately protected, and might be accidentally disclosed or stolen by hackers. In this day and age of cloud-hosted platforms and increased regulatory scrutiny, security of products must be built in from the very beginning, not added as an afterthought.