Here's a list of the actual reasons why mass surveillance of NSA, GCHQ etc are problematic:

No one knows how the world will look like in 20 years time - laws could get changed or even become irrelevant. The main danger here is the creation of a surveillance state. Sadly the majority of people doesn't understand that HISTORY CONTINUES and still think they're protected by “law&order” in their safe and static world.

One could use the data for rating people by threat-potential using AI that analyse the data. (Note that machines don't “understand” context (or for example irony) as humans do nor is such usually unambiguous). Being rated high can have very real consequences such as personalized extended surveillance, being denied access to various parts of society, random searches etc. For instance the German sociologist Andrej Holm was subject to months of personalized surveillance (having people listen directly to all his phone calls etc) and conviction because he googled terms (“gentrification”) that previously occurred in a letter of a terrorist organization ¹⁾. The ADVISE project ²⁾ seems to be an attempt of implementing this.

One of the most important problems (that is especially emerging from the one above) is the constraint of people's decisions that inevitable follows if they know they're being watched or susceptible to being watched and their actions being recorded. This breeds conformity, adhering to norms and constrains the freedom people ought to have by privacy ³⁾. (Also note that the gradual establishment of adhering to specific informal norms makes those norms prone to becoming formal laws. Especially as the eavesdroppers feel their enormous power “Power corrupts; absolute power corrupts absolutely”.)

Also the data could get into the wrong hands - ie one of the ~35000 employees ⁴⁾ (especially private contractors as Snowden was ⁵⁾) could for example sell data for money/private benefits or abuse it because of (ie extremist) ideology. Well besides of the abuse that is already occurring such as “Loveint” ⁶⁾.

Moreover the definition of what is “terrorist” could change dramatically - ie to everyone who's saying something critical about the current norms/institutions/US on the net. For this see also Kissinger's definition by which according to his own words ⁷⁾ “terrorist” refers to anyone “who rejects the international system” (please note that one might reject THE international system without rejecting A international system). Besides of course that Greenwald's journalistic Partner David Miranda was already detained as literally “terrorist” ⁸⁾.

Furthermore economic & diplomatic espionage is already occurring ⁹⁾ which gives the ones eavesdropping unfair advantages ¹⁰⁾ and the eavesdropped various problems. This also includes paralyzation of (mainly economic) progress ¹¹⁾.

Also the whole infrastructure, economy and security doesn't get protected but made vulnerable for attacks from various sources (ie cyber-criminals or China) by backdoors the NSA is installing in software and (ie encryption)mechanisms [or requiring companies to install them or preventing them to fix them] ¹²⁾

Especially during crises the abuse of such data could be critical and the defining moment after all (ie one could eliminate economic competitors deliberately during economic crises for competitive advantages on US side or mass-convict people who are likely part of the Occupy Movement etc). There's already a database in existence that includes of millions of people considered “possible threats” who might get tracked or detained in a time of crisis ¹³⁾. Imagine if this was available during Hitler's times - the “Final Solution” would have been a matter of days.

The main problem however is that they have data (most of their online activities including posts on 4chan for example) on every internet-user ¹⁴⁾ that they could use for blackmailing or character assassination (ie for retrospective justification). That's of course leaving out that they are already spying on users' porn habits for exactly this purpose already ¹⁵⁾. Having sensitive data on every internet-user equals immense power which according to Snowden is what they're actually after ¹⁶⁾.