I am not a lazy teacher.  I do not have low expectations.  My students aren't failing the standardized tests.  However, I don't want my value as a teacher to be determined by a merit pay system. Here's why:

1. Ambiguity: It's too hard to measure "quality teaching" in a quantifiable measuring system.  Sure, we can create rubrics and point schedules that move beyond "value added" scores. However, the minute we try and turn something qualitative into a quantitative measurement, we run the risk of being entirely subjective.  For example, my district offers a rubric for RIF.  However, after adding the scores and comparing them to real-life situations, it was clear that the rubric did not reflect the reality of what makes someone a good teacher. 
2. Instruction Will Suffer: In most merit pay models, test scores play a huge role in teacher salaries.  (I recently wrote a post criticizing standardized tests) Students need differentiated instruction, critical thinking and creativity. A standardized approach will force teachers into a pedagogy that is low-level and irrelevant.  In other words, we'll have more kill-and-drill teach to the test classrooms.  
3. Motivation: The best way to motivate a quality teacher is to offer a higher baseline salary (a living wage would be nice) and then allow for challenges, autonomy and meaning to thrive in a supportive environment.  Great teachers didn't go into the profession for the money and they rarely quit because they weren't rewarded financially.  Instead, the quality teachers I've known have left the profession due to toxic environments, a lack of personal autonomy and greater push toward standardization.
4. Fails to Grasp Holistic Learning: Ask anyone about their favorite teacher and you'll hear stories of a teacher who was a mentor, a counselor and a guide for life.  They were coaches and club sponsors, tutors and facilitators as well as quality teachers in the classroom.  A merit pay system fails to grasp that teachers are in a holistic, human system where learning is not confined very easily to the boxes on an evaluation rubric.
5. Market vs. Social Norms: Most teachers are not motivated by market norms.  They don't view the profession through an input/output spreadsheet and for all the talk of mission statements and SMART goals, few teachers feel deeply passionate about such concepts.  Instead, they are motivated by social norms - the community, the sense of belonging, the chance to make a difference, the deeper existential meaning they find in what they do.  Merit pay is a market norm fix to an issue that is deeply social. 
6. Subject and Grade Differences: Core curriculum classes are different from CTE / vocational, arts, physical education and music classes.  Similarly, teaching five year olds is nothing like teaching seniors in high school.  (Now freshmen, yes, but seniors, no.) How will merit pay deal with these differences?  How will they deal with tested versus non-tested subjects?  How will they evaluate teachers whose roles are so different from one another and still claim to represent a uniform, equitable standard?
7. Context Matters: Gifted isn't the same as special ed.  ELL classroom aren't the same as English language proficient classrooms.  Socioeconomic background and regional contexts make a difference as well.  Similarly, rural, urban and suburban schools each face a subset of issues unique to their populations.  I am doubtful that the proponents of merit pay will allow the local politic to determine a fair way to apply merit pay.  Instead, most districts will be left trying to make sense out of a large, Federally-mandated program.
8. Innovation Suffers: Merit pay rewards teachers who use "best practices" and produce higher test scores.  As teachers feel the pressure to earn a living wage, they'll be less likely to try new practices that could potentially transform learning.  
9. Merit Pay Encourages Cheating: What does Wall Street have in common with Major League Baseball?  When high-stakes extrinsic rewards were offered for short-term gains, people began to cheat.  My hope is that teachers are more ethical than that, but a merit pay system sets up an unnecessary temptation that didn't exist before.
10. Expensive: Suppose we create a qualitative measurement that will determine merit pay.  It includes every necessary variable and does so in a way that is ethical and thorough.  If this happens, we will need supervisors of these systems, trainers of supervisors, conferences for these supervisors, resources for these conferences, consultants who guide districts through the hoops.  In other words, companies like McGraw Hill will siphon off local resources in the name of merit pay - all for an idea that is unproven and most likely will be ineffective at best and most likely detrimental to student learning.