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Firstly...there *IS* reasonable cause to question the global temperature record.  For two basic reasons:
1) The people in charge of collecting, analyzing, and statistically manipulating all of the station data that goes into calculating the global surface temperature record have demonstrated that they are not above using clever statistical machinations to make the data say what they need it to say.
2) If you actually delve into the process used to attempt to account for the inherent biases and errors in the surface data, you will find some very unsavory things.  Gridded HadCRUT temperature data and NASA GISS monthly data both undergo a process called homogenization where, to adjust the temperatures at one station at points that seem statistically strange (i.e. large apparent jumps occur in the annual record that may or may not be linked to changes in the position of the thermometer, the surrounding environment etc), several surrounding stations in the grouping of acceptible long term temperature records are used...the trend at one location is made to fit with the trends at other locations.  This might seem like a good idea in theory, but in practice, it's only a good idea if your network of thermometers is dense enough that the surrounding several stations have similar climatologies.  Unfortunately, the thermometers that actually get used are only the longest records...about 1200 stations to explain the entire world...which means that the nearest five stations to your thermometer could be 500, 1000, 1500 km away in completely different climate zones.
Beyond that objection, a survey of just the US temperature record reveals that over 3/4 of our thermometers don't meet NOAA/NWS guidelines for the placement of the thermometer.  They're often technically over grass, but it's right next to a paved airport runway, and they rarely have the necessary radiation shield or Stevenson Screen.  It's quite appalling actually.
Beyond even that objection...even if we believed our numbers, the magnitude of temperature increase (about 0.7 C) is barely larger than the uncertainty in a single temperature measurement at a single thermometer site (0.5 C even in a good thermocouple and most of these thermometers are still mercury in glass in the poorer nations and thermocouples here in the US that were last callibrated many years ago LOL).  Unfortunately, the IPCC reports claim uncertainties in their global surface temperature averages that are IMPOSSIBLY small...something like +/- 0.05 C.  And that's for gridded data that's been both homogenized AND interpolated to cover parts of the world where there isn't any data (e.g. Antarctica, where exactly ONE station is used...and it's a station on one of the northern-most peninsulas near a scientific research outpost...that statio is made to represent the whole continent??)
Beyond THAT objection...there's the biased distribution of thermometers...disproportionately many temperature records occur in areas that have become increasingly urbanized.  If you search YouTube, you will find a very simple scientific experiment done by a SIXTH GRADER revealing that a simple comparison of non-urban with near-urban sites that are close to each other in the US data set makes the urban heat island impact very apparent, especially in overnight lows and in northern latitutdes.
With that having been clarified...
The skeptic camp is loosely divided in about four branches (and this would be pretty similar to your 2-6):
1) You believe temperatures have increased, but the anthropogenic portion has not been scientifically proven.  There's a reasonable case here as well.  CO2 is definitely a greenhouse gas...you can measure this in a lab.  However, it's not at all clear that the climate system has a positive net feedback in response to increased CO2.  In fact, the paleo-climate record reveals about an 800 year lag between temperature increase and SUBSEQUENT CO2 increase (before the 20th century, CO2 has *NEVER* driven temperature changes...a warmer ocean body is less dense and less able to hold dissolved CO2...releasing far...FAR more of the gas than humans ever could into the atmosphere during many climate epochs).  What we do know is that our current climate models are completely wrong about how the Earth's radiation budget should respond to increasing CO2 in the short term.  Data from CERES (a NASA-funded satellite program) suggests that as the Earth's temperature increases, the outgoing radiation Earth emits also increases...climate models claim that a doubling of CO2 will result in a DECREASE in outgoing radiation due to other feedback mechanisms decreasing planetary albedo and this has thus far not been accurate.  The question of whether CO2-warming will have the net effect of raising planetary temperatures long term is not solved.
2) You believe that CO2 can warm the Earth some, but that the signal we've seen thus far can't have been very large when empirical analysis of other potential climate forcings like the oceanic oscillations, solar-climate variability, urbanization and land use changes and cosmic ray/cloud interactions can be shown to explain a large portion of recent temperature fluctuations.
3) You believe that humans are an adaptable species and that even if global warming did continue as a result of CO2 emission, it might be a net POSITIVE for us...some relevant research suggests that net primary production in the oceans would increase under warmer conditions (meaning more CO2 would get eaten by photosynthesis done by plankton and kelp and the like), and that the growing seasons might be longer in some crucial areas that could help increase food production for the world's population.  Still others have suggested that a warmer Earth would also be a rainier Earth, which is good for humanity.  And this is nto far fetched.  During the Medieval Warm Period, when planetary temperatures were likely at least as warm as today (if not warmer), food production was BOUNTIFUL compared to the centuries before 900 AD when the planet was a bit cooler.  Also note, biodiversity was much greater during the Cretaceous warm period (the time of the dinosaurs when there was absolutely no ice on this planet except at very high elevation and average temperatures were about 12 C warmer than today).
4) You believe that the UN and various nations of the world are attempting to use global warming to scare the people into one global government run by an unelected olegarchy that controls the global economy and has designs on reducing global population because of fears that we are unsustainably overcrowding the planet.  The political side of the Anti-AGW crowd has some overlap with the scientific side but does not count me as a member.  I think that while it's possible some in the UN and in the scientific community express thoguhts like the above (I've read books and seen quotes in the ClimateGate email leak that suggest this)...the vast majority of AGW-proponents are in this because they really believe global warming is a major threat, and the odds of one global government ever becoming reality are so low as to make this concern meaningless.
If you want to see a full scientific review of climate science from the skeptical position, I recommend you google "The Climate Skeptic's Handbook"...the second version of this, which includes analysis of ClimateGate BTW, is available for free in PDF form and neatly organized into the few dozen specific scientific and policy-based objections that have been raised about AGW orthodoxy.