Playtesting sounds like its a great thing to be involved with, and it can be - its certainly invaluable to writers/designers - but its not all glory and roses. Some questions to pose to yourself...
1. Can I work at the pace the writer/designer needs? This is crucial. If you can't get your group together until next month, and the writer needs feedback next week, then you're not adding value.
2. Can I playtest objectively? Some playtesters are of the mind that they're there to trash every rule and offer their own, subjective opinions, offering derisory comments on rules they might not agree with (whether they're broken or not) or in a way that rankles with the writer. If you can't be unbiased or objective, you'll become unpopular very quickly.
3. Am I playtesting actively, or just reading the rules and commenting? Sadly the latter is often the case with little or no in-play testing actually happening. The way a rule is written down might play out very differently in practice. If you can't play, then your value's limited.
4. How disciplined am I? You need a standard approach and a consistent format for how you playtest. This often means setting-up artificial scenarios to test a particular mechanic rather than just running an adventure in the traditional way. Sometimes players aren't comfortable with that because its changing what they're used to.
5. Can you take a rule that you fundamentally disagree with and still playtest objectively? One of the RQII playtesters simply hated combat manoeuvres and flatly refused to use them. You have to see beyond your personally prejudices and look at the bigger picture.
6. Can I give articulate, detailed, objective feedback? Saying 'This Sucks' or 'This is great' just isn't good enough. You need to support your criticisms and praise with examples born from the playtest process. Otherwise, the writer has nothing to go on to make improvements.
7. Can I ignore typos, grammar errors and so on? You'll be working from an unformatted manuscript a lot of the time. Its not your job to correct spelling errors, punctuation and so on, but you'd be surprised at how many playtest reports are simply lists of minor corrections and not a playtest report.
8. Can I avoid making comparisons with previous editions and other systems? You're here to playtest what the writer's worked on - not spell out how version X did it so much better, or compare the game with your other favourite system. Sometimes comparisons can help illustrate a particular point, but should be used sparingly.
Playtesting can be time consuming, arduous and often a trial rather than a pleasure. Its something you do for love and commitment rather than riches and glory!